Best way to prevent side panel vibrations?

[lrlcr]

I've just finished watercooling my 800d (here) but it has a really annoying buzzy/whiny noise coming from pump resonance. If you press on the non-windowed side panel it seems to go away so presumably I need to damp this.

What is the best way to do this? Is the acousti matting any good? Is there a cheaper way to do it effectively?

Help greatly appreciated as the noise is doing my nut in!

 

[lrlcr]

I'm not sure there is space for that, and how is it at damping direct mechanical vibrations?

 

[lrlcr]

Unfortunately not, it's in a bay res which has rubber mounting feet however it does seem to transmit a load of vibrations into the case still. Fixing that will be an ongoing point for thought.

The really annoying noise stops if I poke the side panel in appropriate places though.