Walk behind Lawnboy - making some noises

Regarding my recent purchase of Lawnboy 10343, serial 4906322, 1994 model, Green Series.

This mower runs really great when I am mowing the lawn and the self-propelled really pulls it along. However, when I let go of the bale for the self-propelled, it makes noises. As soon as the bale is pulled for self-propelling, the noise stops.

It doesnt sound like it needs a muffler. Anyone have any suggestions on what the noise it? Thanks very much.

I would take the belt cover off and look to see if you have any loose parts rattling.

Fireman, many thanks. It was the cover. Running very good now...no noises. Thanks again.