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???
Thanks,
Warren