Misterious noise in a 737

[LEM]

On one of our 734's a very strong noise, quite similar to that of an electric hyd pump, develops under the left overwing emergency exits while in cruise.

It's really strong, and in the troubleshooting we've made we found out it's N1 dependant (it is higher around econ cruise setting), does not varies changing bleed or pack configuration.

What the hell can that be?

Pax are complaining, but our ground engineers don't seem too concerned about that, and I'm afraid they are not going to ground the airplane for a deep troubleshooting.

Engine vibration indications are normal.

A resonating valve in the air ducts maybe?

None else ever experienced the same?

Thanks!

[Dr Illitout]

I recall hearing of an odd noise on a 737 in the cruise that they traced, after a LOT of messing round to the bearings on the thrust links. I'll ask around
Keep us posted on this one

Rgds Dr.I.

[BOAC]

Did you check the recirc fans? I have a feeling I had this a long time ago and it was a 'dicky' fan. I think the supply duct enters where you describe it.

[Notso Fantastic]

I remember on our 737-200s, the air ducts could produce some very bizarre rumbles/roars, particularly around the FD/Fwd toilet area. Maybe a bit of investigation turning off recirc fans and even packs for 30 seconds might reveal the culprit.

[Eric Janson]

I do remember a similar problem in one of our 737-300's.

High pitched buzzing noise in flight. Initially everyone thought one of the overwing exits had an air leak.

Problem was traced to a loose wing-to-body fairing that had been removed during maintenance and then not re-attached properly.

Temporary fix with high speed tape solved the problem.

Hope this helps

[LEM]

Yes I tried switching off the recirc fan, the hyd pumps, the bleed, the pack.... uselessy.

The engineer I talked to after the flight excluded a bearing problem in the N1 shaft...

My bet was a loose fairing, but....

Nobody seems to have a definite answer.

I think an engine bearing would disintegrate before producing such an awful noise, and anyway the noise seems to be generated int he wall itself, close to your left foot if you sit there, as I did...

I hope somebody will tell me when and if they solve the problem... Thanks everybody

[gas path]

Try to get the wing fuselage/fairings checked again and to check that the panel gaps are not too big, we had a triple seven that suffered from resonance and vibration around the wing root area and although the panels were properly secured, speed tape over all the joints cured the problem. finally cured by adjustment and sealing of the panel gaps.
Mind you we also had one doing the same and found an empty but rather battered mobil jet 2 oil can rattling around in the mix manifold

[fruitloop]

Ask the engineers to check the Ram-air door actuator linkages and bearings.

[DDG]

Airbourne Vibrations B737 Classic

LEM, POSSIBLE CAUSES OF YOUR VIBRATION

1) MAIN ENGINE CONTROL / FUEL PUMP this is a known problem with cfm56,a resonating imbalance between 3d cam/speed control weights and the rotating mass of the fuel pump impeller due uneven bearing wear within the main engine control.Can be confirmed by varying thrust and vibs will vary in pitch & strenght.This is usually felt through the flight deck floor. Final resolution is to change the MEC.Boeing/CFM have a service bulletin regarding the matter.This usually appears on a high hour engine.

Regards DDG

[LEM]

Can be confirmed by varying thrust and vibs will vary in pitch & strenght.This is usually felt through the flight deck floor.

Yes varying thrust the noise comes and goes.
No, we don't hear anything from the flight deck, you must be within a few seat rows from the precise point where the noise is generated (under the left overwing emergency exit).

But anyways DDG probably you found it as it seems to depend on thrust changes, not airspeed changes.

If I have the opportunity I'll check again, now that I have a more precise idea thanks to all your replies.

Thanks again.

LEM

[Suffering Sucataash]

LEM,

Are you flying in humid conditions, could it possibly be the ice separator associated with the left pack?

I had an awful shuttering noise from this area one night and subtle fluctuations/fluttering in cabin pressure which from memory may have been the separator just getting the right amount of ice so that it didn't know if it was Arther or Marther. (Classic 737)

SS

[Notso Fantastic]

LEM- final desperate suggestion. This is the location of some cabin speakers. We used to be troubled on a few of our -400s by feedback type noise emanating from some speakers in the emergency exits area. I do know that the actual source of the noise can be completely impossible to pin down, but maybe pulling PA CBs might help.

[mainwheel]

Lem,

If you get the chance, have someone sit in that row, and cycle the L/H Fuel boost pumps.

The elect HYD pumps have a resinator type accumulator that could explain it, cycle the pumps again as well. At least a minute.

Any feedback to your guys is helpful. And appreciated.

[eng1170]

Some a/c have an APU fuel pump that sits in the falsework in this area, but wouldn't account for the noise change with N1. Or the triangular panel over the landing gear bay, I've found a few missing fasteners in these before and I know our company removes them alot.Engine hydraulic shut-off valve is also in this area but you'd probably have other symptoms/warnings with this. Or there's the possibility the gear door(s) aren't faired properly, it's not something that would be immediately visible on the ground. As already mentioned, an aircon snag of some sort, pack valve maybe.

Just a few thoughts

Eng

[autoflight]

Don't ever give up your efforts to find the noise. If it is not identified, there can be no assurace that the aircraft is airworthy.
I made many reports of slight vibration in an A320 over a 6 month period. After 2 inflight diversions back to base, the company finally got the message. Grounded the aircraft for investigation by an airbus engineer who found extremely serious damage where the horizontal stabiliser attaches to the aircraft. Of 4 essential bolts, 1 was broken and 2 loose. One bolt separated the crew and pax from eternity.

[tewkesbury]

Try the Hydraulic reservior presurusation module, situated on the roof by the overwing exits, if the reservoir over-pressusises it will cause a chattering of the the pressure relieve valve, which can be heard in flight, and as with a lot of problems is difficult to replicate on the ground.

[747FOCAL]

Can you produce the sound on the ground? If you can produce it on the ground it is not an aero noise and most probably an N2 noise.

[eng1170]

1. carry out functions on hydraulic systems in area to try and replicate the noise, (gear swings and check door fits as well)

2. carry out cabin pressurisation check to rule this out, also packs running during this will allow these to be ruled out.

3. engine ground runs, ideally suspect side at higher power.

4.airtest with eng on board!

[avioniker]

At altitude and at higher power settings may be a leaking pneumatic or muscle line in the interstage area. The 9th stage may be overcoming the shutoff and whistling. A very small leak will be VERY loud at altitude and since it could be in the pylon or very near to it the Pax will be getting the best benefit of the malfunction in their auric canals. (wow I didn't know I even knew that word)
You won't see a pressure problem on the overhead as the PRSOV should take care of that small matter.
They should be able to duplicate the leak on the ground but the noise won't be as noticable because the fan is making its own noise in sea level air.
I've heard the difference between a CFM at power at sea level and one in Ft. Collins. It's pronounced.

Lots of luck on this one.
Let me know

[LEM]

Unfortunately, with more than 30 airplanes now in the company, God only knows when I'll be flying again on that one...

Thanks everybody for the suggestions.