[Jim Koehler]

You could be having shielding problems. Make sure your RF shield cans are properly grounded. Also, a small capacitance mica cap in the RF section may be going defective. 

That mechanical hum could be either the power transformer is not tightened down onto the chassis or the laminations from within the power transformer are becoming loose. I've solved the latter problem by heating the transformer up and in doing so this warms up the varnish to a point that it gets soft enough for the laminations to "restick" themselves together. Makes sure that the binding bolts are good and tight.