I don't know about the relay pop, you may want to make sure that the amp and AC outlet it's connected to are properly grounded. Beyond that, if the amp produces audible hiss without the inputs connected and the speakers aren't super high efficency models, then you'll need to send it back for servicing. There may be an internal problem. Try pulling the AC plug and then repowering the amp after about a minute. Sometimes that will allow the regulators to reset, which might get rid of the noise.
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