Flew on a United A320 last week which made a strange sound during taxiing. At first I thought it was a lose link in the U/C making a clunk, but then I recognized the sound as a panel 'oilcanning' i.e. a nearly flat panel with a small distortion that pops back and forth between one shape and another like the bottom of an old-fashioned oil can as stress changes direction.

I was seated 4 rows behind the overwing emergency exits, and the sound was coming from below me. It was quite loud and distinct.

Is this something normal for an A320?

I've heard this from the same position. It seemed to coincide with brake applications, so I assumed that the noise was being made by a valve in the braking system.

What do you think the skins of airbusses are made of anyway?

Pax sitting around that area often comment on this noise.
It's either the brake system shuttle valve operating, which makes a kind of 'clonking' sound. Or if taxiing single engine, the Hydraulic PTU (power transmission unit) may be running, this sounds more like a 'whining noise', starts off low down & builds up to a crescendo, then stops, then repeats the whole process over again & again.

The shuttle valve could be it; I only heard the noise during low speed taxiing. I didn't hear it during either takeoff roll (two segments in the same aircraft), nor rollout.

Definitely scary stuff...Boeings don't do that....I was wondering why all the oil companys went to plastic containers..Airbus bought all the tin.

777 - Tch, scaremonger. All aeroplanes make scary noises when you don't know what's going on. It's when they make scary noises when you thought you did know what's going on that's more worrying.