Neither a particularly high or low note. Right in the middle of the range, slightly to the left of the exact center of the keyboard.

If you're absolutely certain there's no distortion in the headphones, then it's either the speaker, the amp, or cabinet rattling.

Try getting your head down between the two speakers if possible and listen to the playback on repeat. Try to narrow down exactly where the distortion is coming from.

If it's only coming from one side: Bad speaker or physical rattling/buzzing likely (or, less likely, just one channel of the amp).

If it's coming equally from both sides: either it's physical rattling coming from a different place than the speakers themselves, or, it's a bad amplifier.