Fault finding
Finding the source of unwanted audio signals heard in the phones needs to be systematic. You may be fortunate that the "noise" is present all the time which makes pinning down its source a lot easier. There are some obvious questions to be asked here like; does this fault occur with the engine off? Is it present on both pilot and passenger audio circuits? What avionics do you have fitted? Did you have any problem prior to the Annual? Was any electrical or radio work carried out other than the normal Annual testing/calibration.
Some good suggestions have been offered already with regard to the additional nav/com gear that is fitted to your helicopter. It’s a matter of isolating everything, including the audio panels and seeing which one of the black boxes is playing up.
With the engine off and the noise still present, then this tends to eliminate the engine side of things at this stage. Switch off all transponders, nav and comms units at their individual on/off switch. Switch both audio boxes to their emergency positions. This now isolates the internal audio amplifier of the audio boxes and connects the pilot’s mic, phones and ptt to (usually) comm 1. If the noise is still present, you have an external wiring problem not associated with any of the black boxes. Talk to the engineers that did your Annual and find out whereabouts in the airframe they have been playing.
If the noise is not present, with all the black boxes still off, switch the audio panels one at a time off the emergency position and to comm1. If the noise is present, switch each off in turn to see which audio box is the culprit.
If the noise is not present, switch all the receive audio switches on the audio panel to your phones position then switch on comm. 1 at it’s on/off switch and check. Ensure that the comm’s squelch control is not open. Some comms have a pull up function on their volume controls that open the squelch, which is easily missed that it is sitting up. If still no noise that would have indicated that comm1 is the villain, turn off comm. 1 and do the same test for comm. 2. Remember to turn off each item once it has been cleared as individually being the cause.
If noise is not present, switch off comm.2 and try each of your nav units individually in a similar manner. Finally try the transponder. Then turn everything on and see which if any cause the problem to reappear.
Some other equipment can feed direct into audio panels without any means of isolation. Things like Rad Alt and AVAD and other audio warning system in the aircraft that you normally hear in your headset could also be a source.
Good luck.
Geoff