Treadmill Noise?

I just assembled my Proform 580. It went together easily and runs well. The belt is centered and does not appear to be slipping. I finished the first 30 min. excercize without anything unusual except some intermittent noise.

The noise occurs only if I walk on the left of center and towards the front of the walking area. I'm not talking about some grossly out of balance positioin, just walking more toward the left side. The sound, to me, would be of a slipping belt but I cannot detect that in the walking. It is loudest at foot strike and tapers off as my foot moves backwards. The noise volume is not very loud.

Any ideas?
Thanks
Russ