We discovered an air filter was never installed in a house we acquired. The house was flipped three years ago and the filter was never installed. When I installed the filter, i started to get whistling noise, so I called Alan (he answers all of the phone calls!), He diagnosed the problem over the phone - he was sure it was because of the dust that had accumulated in the furnace due to lack of a filter. Alan promptly sent one of his service people who cleaned out all of the accumulated dirt. Not only did the whistle go away, the furnace is running much better now. I could not be a happier customer with the great service, prompt attention to the problem furnace, and quick resolution.