By any chance are you using a tube preamp? You should always power up a tube preamp and wait a few minutes before turning on your SS amp. Tube preamps often emit strange pulses until they warm up (solid state ones sometimes do the same, but immediately upon turn-on, and disappearing quickly). If you wish to be sure this is the problem, see if you get the same noises turning on the amp while the preamp remains off. Proper power up sequence is front end to amp (CD player or phono stage first, then preamp, then amp). The turn-off sequence should be the reverse.