I have a Trane XLi19 A/C unit. Installed in August of 2005. It has been making a gurgling sound inside the air handler since first installed. It only does it after it has been running on low speed for awhile. If it kicks into high speed the sound goes away and when it shifts back to low speed it takes awhile for it to return but stays as long as it remains in low speed. The gurgling noise gets distributed throughout the duct system and is quite annoying. I had it checked by the installers but they couldn't replicate the sound so sort of blew me off but sent me a bill for $100.00! I've been living with the noise ever sense but I am really tired of it. Can this be remedied (what's the most likely cause/fix) or is this just a characteristic of the two speed a/c and I have to live with it? Looking at my invoice, the coil is TXC037S3HPD/5275XBE5G, the A/C unit is 2TTZ9036B1000AA/5131XCW1F, the gas furnace is TUY100R9VW5/5084ML77G.