On the left side, there a plastic cover. I think it's the belt cover. When the motor is running
making a knocking noise from inside cover. I can pickup or push down and make the nosie go away.
Is this a simlpe fix? (maybe installing new washer or replacing cover).

 

It is defintily the clutch cover. Try loosening all the bolts around it and get the cover centered. If you let it rub too long it will wear a hole thru the cover and then it won't be water tight.

And welcome to the forums