I've got this Marshall 18W TMB two-channel amp. Channel one has no issues in and of itself. The TMB channel, on the other hand has this very specific rattle sound when I play anything near D3 (146.8 Hz). At first, I though it was a cabinet vibration. I eliminated that. I put the chassis up on the test bench, made sure all the screws/nuts were tight, made sure the tubes were seated, the pins were making contact, and the tube clamps weren't rattling. So, here's the deal. When I play a D at moderately loud volumes, I get a sound that sound like a physical vibration, but it's coming through the speaker. I pulled out my chopstick, and put my signal generator at 440Hz, then 220Hz and then 146.8Hz. At 146.8Hz everything I touch creates a rattle through the speaker. Every jack, every resistor, every capacitor, when I tap it with the chopstick, I get a buzz/rattle coming out the speaker; it sound something like static. It's not totally specific to 146.8Hz, but it's in that neighborhood. You definitely notice it playing guitar through there every time you play the 4th string open D or 5th sting at the 5th fret D. I'm still looking, but at this point, I haven't figured it out, yet. Has anyone had a similar experience.