Whirlpool Washer smell?

I woke up and my mother in law had stated that her 6 year old whirlpool top loader washing machine had a burning smell a few minutes after she had just put a small load in to wash. I went to check it out and ran it through the remainder of the cycle with no problem. I took the front panel off but was unable to locate the belt to see if it needed replacement. I left the panel off for a few hours and then put it back on with no smell anymore...can anyone tell me what's going here?