[Music in a bottle]

I just realized I'm in the wrong part of town! I have a Marantz model 15 Dual Monoblock Power amplifier. The units both power up and play fine. I have them hooked to a separate preamp, and upon hitting the preamp power switch, the amp makes a loud and violent enough thump at that moment of hitting the switch on the preamp, to rattle the top cover of one of the mono blocks! Both mono amps are bolted together behind the face plate and each of course have their own power supplies and cords, but one amps ac power cord typically plugs into the other module as one has a long cord and the other a very short cord. In the owners and repair manuals I have, they clearly state that there is no mechanical relay to speak of. The long delay at power up before you hear any sound is achieved through a series of diodes and resistors. The amps have the usual speaker protection measures such as zener diodes, that divert excess current to a bank of red lights, on each module, with the bulbs dissipating the current and giving visual indication of possible overdrive, excess DC offset or a short etc. I currently have no speakers hooked up, but the red overload lamps I speak of (2 per module) have never lit up. These overload lamps are also used to adjust for zero DC offset. The DC potentiometer located on the rear of each module is rotated until one of the two red overload lamps just begins to light or flicker. The screw is adjusted the other direction until the other red overload lamp begins to light up. Correct DC offset is between these points. Both modules are presently in the original wooden cabinet that houses both modules, and all this is in a rack type cabinet. I really need to pull the unit from the cabinet and separate each module, then power them up to determine more accurately where the mechanical thumping is coming from. There are 4 diodes off the transformer, that are attached solely to a small board on a bracket above all other components, just below the top cover. I noticed long ago, that there was a slight black film on a couple if them that could be wiped off. Additionally, the rather large zener diodes in the speaker protection circuit are bolted to studs on the heat sink and I noticed that a couple have what appears to be diodes? Soldered across the connections. Some of my parts modules have the same type diode looking components across the same zener diodes. Not sure what could cause such a mechanical noise, that would not come through the speakers, but I'm guessing a power transformer would be the only thing that could have the sort of mass to do such violence, but certainly have considered those rectifier diodes too. Was contemplating testing the transformers as I have an original service manual that shows the voltage at each lead, but was also more likely considering sending this to someone who knows what the heck they're doing, because I don't!