My Crestron is in a fire rebuild house. The last six months has involved lots of house finishing and customization. I only accidentally was able to work on this problem yesterday, it being lower priority than other issues. (The "accident" was that I wanted to debug a start-up problem that I thought was due to my circuit for use without a controller. It turned out the Crestron had popped a 20 A breaker. So I need not have had the cover off, but while I did....)
I am happy to report, for the curiosity of Slip and Michael and possible future search users, on the solution. I tightened the bolts securing both transformers by 30 degrees each. The hum went from noticeable to not noticeable.
FYI, in the 16x60 version of the Crestron, the bolts are horizontal, and thus could not have been touching the top. There was no evidence of mistaken repair. Both bolts seemed to have the same feel, or resistance to tightening. The transformers are held in place with a compliant (rubber?) disk and a metallic disk. I would think it would be easy to overtighten the bolt and distort the metal disk, so I am glad that a modest tightening was adequate.
