Assuming you hear it from the speakers:
1) If you can hear it at normal listening distances and levels, it sounds like the receiver has a problem.
If so, it's time to visit a tech or replace it.
2) If you have to put your ear to the speakers, it's most likely normal.
I include both options because your complaint doesn't specify which...
3 If it's coming from the receiver itself, it's most likely a power transformer on the way out.
See option 1 above.