Popping sound in the amp

[WY0Q]

I have an Ameritron 811 Amp. This started yesterday. When I turn it on, about a minute in the warming up process, and about every two minutes later, (only ran it 5 minutes)it makes a popping sound, like you would hear if you thumped your fingernail against the front panel. I opened it up, no evidence of arcing, all three tubes appear fine, the blower works fine. I thought it might be some expansion from the heat. It is about 3 1/2 years old. Any ideas?

[W1QJ]

It sounds like a PS capacitor arcing (leaking). The best way to find it is to watch it in a pitch dark room, you'll see it as plain as day when it sparks. Just be careful because the lid will be off the amp. BE CAREFUL.

[AG6K]

It sounds like a PS capacitor arcing (leaking). The best way to find it is to watch it in a pitch dark room, you'll see it as plain as day when it sparks. Just be careful because the lid will be off the amp. BE CAREFUL.

 When electrolytics get sick they often emit a white deposit around their pressure vent. If you find a white deposit around one or more pressure vents it would be a good idea to replace all of the capacitors with better caps. Matsushi-ta's (Panasonic) low ESR, 105ºc line of electrolytic caps are excellent but they are not cheap. Mouser and Digikey carry them.
If the problem is not the filter caps, the noise might be coming from the xfmr.