Ashdown distorts when playing a D

[Eskimo Spy]

I have an Ashdown ABM EVO II C410T combo, and when I'm playing a D, fretted or unfretted, it doesn't have the same clean sound every other note does. It sounds like it's distorting just a little bit, but it's not significantly distorted, just enough to be noticeable.

And here's the odd thing; when I lean the amp forward, it stops the distortion. Put it back flat, back it comes. It doesn't seem to be affected when I lean it backwards.

Any ideas?

[Eskimo Spy]

Okay, it was one of the screws that held in one of the lower speakers, it was tight, but it had the speaker right up against the wall of the amp. I loosened it, move the speaker over slightly, then re-tightened it, distortion gone!