Hi @dcstevens ,
Have you tried different inputs to hear if it only occurs on 1 input or if it occurs on all inputs?
Here is a link to the service manual for your receiver.
To download the manual:
When the sentence below the Document preview box changes from This file is downloadable free of charge: ...processing.. to This file is downloadable free of charge: Get Manual click on the Get Manual as this is the download link. Be patient it takes a little while to find the file.
The service manual has everything that you expect in a manual, with circuit block diagrams, wiring diagrams, test sequences and a parts list.
You state that "nothing broke anywhere on the receiver". Have you checked on the inside?
With the power disconnected from the receiver, open it up and then look for any obvious damage caused by falling and hitting the floor, such as cracked pcbs, components broken etc, loose connections, components shorting out to the case (on the inside) etc