In a quiet room late at night, standing several feet from the RR2150 which is not enclosed in any cabinet, I hear a slight buzz (which I assume is coming from the transformer), not coming from any speaker, and separate from any volume setting or any other setting on the receiver. Is this normal and how many db over ambient level should such sound not exceed?

I have a db meter. If it would help, I can give you the db reading above ambient room noise with the amp on and off.