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Possibly, or power supply noise from the iMac being picked up and fed into the amp? It increases when the amp goes to sleep suggests when the demand is less on the iMac's internal PSU its being fed down the 3.5mm (I assume you're using that kinda connector out the headphone/line out socket?). Is the iMac definitely plugged into a grounded mains socket? I'm floundering/guessing as sometimes trying to diagnose Electronics faults via webbernet is like performing keyhole surgery with one hand over your eyes sometimes!
Does the amp work OK with other sources like a CD player or batt powered MP3 player? Noise/buzz free?
blushes I feel so out of touch with modern-speake at times!
Ta for for enlightening me TRA and Olly, I feel younger already :)