My 10-cpm convection blower is starting to get a little noisy on the higher speeds. Anything less than 5 is fine, but when I'm running between 5-9, it has a prominent vibration. If I touch the blower housing, it seems to go away and run quietly. The screws holding the blower are all tight. I've cleaned and oiled the blower within the last month. Any thoughts? I'm thinking there's not much else I can do except replace it, but thought I'd throw out a question to the pros. Thanks.