If the unit is completely enclosed within your rack, maybe you can just remove the top panel altogether. Similarly, my Bryston 2B (earliest two rack size, not the later 2B LP version) lid would significantly amplify transformer hum, no matter what I tried the way of damping materials or screw tightening. I decided to just leave it off, with much improvement. But one could only do this if the top of the unit is sufficiently closed off (depending on the rack configuration) for the safety of preventing a hand or fingers from slipping in. Second to this, you could try a piece of plywood (say 1/4" thick) or thick plate aluminium (if you have a metal shop not far from you) cut to shape just a bit smaller than the GT Pro lid, and fasten it with heavy duty double sided tape, secured at the perimeter, and with a large X in the middle, for damping and bracing. You could possibly mount this plate inside the unit, but even with the tape, I would be inclined here to use one small screw, in addition to the tape, just to guarantee the wood will not separate from the top, and possibly damage internal components from falling.

I just remembered another fix I had to make to my Carver C3 Preamp, which also amplified transformer hum. I took the top off, and fitted a square wooden pillar that was the right height such that when I put the top back on, it coupled and braced the lid of the unit with the base. Again, I used double sided tape on both ends. This trick requires accurate fabrication of the pillar length, with perfectly parallel end faces, and some rigid place for the bottom to stick to. Your GT Pro's PCB might prevent this from being a feasible option, but it is worth a look inside, to see if there is some base to work from. I don't think it needs to be dead center, anything more than four inches from the edge might do the trick.

Or RTV sealer. Run a bead all around where the sides meet the lid. If there is room to couple the top and bottom with a pillar then use the RTV to keep that in place as well.

RTV sealer could work, even by itself, but some mechanical rattles seem to defy the simplest fix, especially when they are caused by high energy excitation of say an electric guitar rig complete with multiple cabinets. I think some kinds of rattles or buzzes are caused not just by mechanical action at coupling points or lines, but through sympathetic vibrations of thin panels, or even other parts of the device -at certain frequencies of excitation, thus emitting their own unpleasing sound- in which case, increased rigidity by some new point of lid and chassis coupling (or other loose parts) may help. But RTV sealer is as good or better than anything else to try first.

Whats RTV sealer? Silicon? I haven't brought the rack into daylight to get a look as I'm having new storerooms built and everything is pretty inaccessible atm, but I think the Boss lid is not removable. I'm going to fix in some thick foam I think and permanently fix the wires into the back of the MXR1500. Simple is best, I guess.

Room Temperature Vulcanizing. AKA gasket maker. Used as a sealer, gasket, or gasket assist in auto engines. Certain formulas withstand very high heat.

TheJackAttack, post: 362355 wrote: Room Temperature Vulcanizing. AKA gasket maker. Used as a sealer, gasket, or gasket assist in auto engines. Certain formulas withstand very high heat.

They also STINK !!! That's one sure way to keep the bass player AWAY from you...LOL!!!