Everytime this unit kicks on it makes a VERY loud noise. Kind of a high pitched groan. You can actually hear it outside the home 1/2 block away! I have had the propane company out 3 times to clean/tune it up. They seem to think it happens because of temperature differences and metal expanding. That may be it but it does happen in both cool and cold weather. Once it is running for a minute or two the noise abates. Any suggestions?