I have a stand-alone tube preamp I like to use while recording. It's not very old and recently it started adding some intermittent clicks to the audio stream. If I plug directly into my mixer instead of using the tube preamp, I don't get any clicks, but if I route through my preamp occasionally it'll add a very audible click, not a series of clicks just one single click, to a recording. Sometimes a recording might not have any, and sometimes there might be two or three in one recording.

I contacted the manufacturer and asked them if it was the circuitry of the unit or the tube causing the clicks and they just said the unit needs service and I should send it to them.

So I ask you, O wise community, what do you think would be causing this?