NAD 752 AV receiver hisses a bit during warmup

[marcplante]

I have an NAD 752 AV receiver that hisses a bit during the initial 30-60 seconds of its operation. It then clears up and seems to work fine. I've located a good service center, but they're not local, and I'm reluctant to pack it and ship it for service unless this is a serious problem brewing. Any preliminary diagnoses?

Thanks,

Living Room Rig:
Panasonc 50" Plasma
PSB Silveri towers and
PSB C5 center channel
PSB Stratus Sub
NAD 752 AV receiver

No rear channel yet