Electronic Interference On Speakers

I have a Cambridge Audio DTT3500 and have used it on my PC with my B&W 601 S3 speakers for about 3 years, and I have a BIG PROBLEM

in weeks gone by i used to have a popping coming from the channels

Now it sounds like a AC sweep signal, it starts at a high frequency and it slowly decreases(It is a audible frequency variation) it goes down to only a few Hz and increases up to definitely a few thousand Hz.

It is not a sinewave, as it sounds like popping.

It is not the PC or speakers as i have disconnected all signal inputs into the amp and still it has the same issue.

I dont have a Oscilloscope and my Multimeter cant pickup any frequencies when i attempt to measure the interference.

CAN SOMEONE PLEASE HELP!!!!

The internal circuits could be oscillating.

It stops after about an hour or so...

amp oscillations like what you are describing can be caused by dried out electolytic caps in the power supply (most likely) or the amp itself (or both). what you're describing is what i'd call a "chirp" more than a "pop" and indicates oscillation (a "pop" would be rapid DC shifts and spikes in the output).