I have a TFM-35 and two TFM-35X amps, and all 3 produce a soft popping sound when the source signal first hits the input. It's in both channels, and it pops softly for about 5-6 seconds (1 pop every second or so) and then stabilizes and dies out. Every once in a while the signal will somehow excite this behavior again and it will stabilize after another 5-6 seconds. Each pop almost sounds like the sound of a stylus initially dropping onto a record, but occurs with all sources including CDs. It also shows up on the power meter needles, so I can see it at the same time that I hear it.
I've tried Carver, Yamaha, and Marantz preamps, and Mordaunt Short, B&W, and home made speakers (as well as subs) and it happens with every combination. All equipment is plugged into the same outlet, so I don't think that grounding is an issue. I've even tried the entire system in 3 other houses with the same results. It's definitely localized to the amps.
Anyone else see this issue anywhere? It's not loud or to the degree of being annoying, but has me curious as to what's going on. They all sound very good otherwise, but this has been happening for as long as I've owned them. Bought all of them used, by the way. Any input would be appreciated.