It would be prudent to place a Periodic Service Call to a whirlpool pro on this one. It sounds like it needs one. W/O seeing it, I'd say that it hasn't been used in a while, and it's got some 'junk' in it. Down the road, you may have bigger problems as a result of this.
WHen you call 3 for estimates, have the make and model handy. (Call the previous homeowner if you need to. The realtor may know where to find them. ANd find out where it was bought too.) Discuss what the Periodic Maintenance includes. After you've spoken to 3 of them, you should have a pretty good idea of what needs to be done.