Funny noise from Tube system

Hi everyone... I have a pair of Rogue Audio M-120 Magnum Monoblocks with the 99 magnum preamp. When I have the system on for about 1 and 1/2 hours I hear a pppppfffft...pppppfffft noise over and over. It comes out of both channels and stops when I engage the mute the sound stops. I think it's temprature related, only because when I turn on everything it is silent. I replaced the tubes in the 120's but have not replaced the 99's as of yet. I bought the equipment new in 2001. Any ideas???
The electrons are revolting against being forced to work with the tubes.
Keep the mute switch on
I would ask Rogue. Since it's not there when you put on the mute switch it's not in the amps. While it could be tubes, it also could be a capacitor or even a resistor which only exhibits this behavior when it gets to a certain temperature. I had a similar problem with my JP80 preamp a few years ago, it took forever to find the problem (was a resistor) because to get to the problem component we had to take off the bottom plate, and when we did that it cooled off the resistor and stopped the problem. A good technician should be able to find it, but Rogue is best equipped to do the trouble-shooting.
I heard the same from my tubed system, but only out of one channel. I checked the tubes in the preamp, and, sure enough they were bad. I replaced them and that PFFFT sound went away. Roughly speaking my preamp tubes are good for about a year. Try changing them out if you con't have a tube tester.
I think it's time for new tubes in the pre-amp....

Pre-amp tubes should be good for 3000 to 10,000 hours, depending on which tubes and which amp???
Depending on your listening habits, you could be well over 3000 hours on the pre-amp tubes since 2001. 10 hours per week for 6 years = 3120 hours of on time. 20 hours per week for 6 years is 6240 hours and 30 hours of on/listening time per week is 9360 hours over the course of 6 years.

Since the problem goes away when you place the pre-amp on mute, you know the issue is not in the amp. I think replacing the pre-amp tubes is the next logical step.