I am having the exact same issue. Curious if you’ve found the culprit?
McIntosh MP100 hissing noise
Just picked up brand new MP100 from authorized dealer and noticed slightly hissing noise coming from the preamp when in standby mode. The noise goes away when unit is turned ON. I’m wondering if any of MP100 owners experience the same. I tried disconnecting all interconnects, but same result. Awaiting respond from McIntosh, but wanted to hear from MP100 owners. Thanks.