this thread is an interesting discussion. i absolutely agree that a professional is better equipped to diagnose an hvac problem that i am. however, the trouble is that the professionals in the trades have all kinds of motivations that have nothing to do with repairing my furnace. there is some good advice in this thread regarding choosing a contractor, but is still really hit-and-miss in my experience. for every good experience i've had with a contractor i've had three bad ones. and i generally use "well respected" contractors. someone above mentioned that the big companies have youngish guys with seemingly little experience who strongly push sales. this has absolutely been my experience. hell, here are my experiences 1. roots clogging a drain - plumber recommends running a scope down the drain. says he'll do it for free. does it, tells me i need a $15,000 pipe lining on 65 feet of clay sewer main. strongly warns against cutting out the roots, he says it will cause the pipe to fail and then he won't be able to line the pipe. i say, no, do it, cut the roots out. he does. that was a year ago and everything has been fine. charges $400. 2. quote for gas line to stove - contractor comes out, looks around for 5 seconds, says, $1300 for apx. 30 ft of gas line to the first floor kitchen. no thanks. no harm no fowl. $45 for the quote. 3. clearing exterior downspout drain to sewer - spends some time determining the pipe is totally blocked. i ask him to quote me a fix. he recommends running the downspout underground in pvc to the curb and dumping on the sidewalk. 30 feet. $1100. i say, "city code requires stormwater discharge go into the sewer system, how much to do that?" "oh really?" he says. "i had no idea. that would be at least double what i quoted you" ----- these experiences are from different large local companies. so you'll forgive me if i default to thinking professionals are more often than not full of it. of course they all aren't, but how do you find the good ones? there is also a general notion that DIY is somehow equivalent to "screwing around." HVAC and the like is basic physics and electronics. I think a patient, well-informed and experienced DIYer can easily do as good of a job as even a good professional. I have been able to fix refrigerators, washers, rewire electrical systems in campers and cars, reorganize spaghetti circuit breakers, rewire rooms, repair plaster ceilings and walls to where pros compliment the work (and say, i would have just torn it out and hung drywall), replace car components, and other "professional" tasks after carefully researching, learning, staying patient, and executing. and in this case i don't really even want to do the work. but it is starting to seem like the least worst option.

I think a single servicing is adequate and should protect the manufacturer's regular service warranty. I do mine in the late summer/early fall. Heat pump and 90% gas furnace. You could also do it in the spring/early summer. I personally think twice a year is overkill and probably a sales job for an additional service call. Unless the manufacturer's warranty specifically requires it, I would do annual service only.

Some people may disagree with me, but I think alot of preventative maintenance is a waste of money. I believe if its not broke don't fix it. I would rather not have people tinkering and possibly breaking/sabotoging my equipment when it is working fine. Its another story if it is broken or not working correctly. Homeowners should replace air filters often. I would recommend only calling for service for a heatpump/ac if the output temperature rise/drop is low or making weird noises. Its easy to test this. Just invest in a digital thermometer from ebay/amazon and stick into the vent when its been running a while.

Did you manage to determine what the source of the noise was, megnmike? We have the exact same issue happening with ours: Carrier furnace; wasn’t always making this noise; the sound is not whistling or whooshing from the vents or whirring/grinding from a failing motor, but a distinct musical tone (a high C#, in fact :) ) that is loudest when in the furnace closet but permeates the entire house through the vents (loudest in those closest to the furnace itself). I’ve tried removing the (recently replaced) air filters and letting the unit run just to see if they were loose and creating harmonic resonance, no dice. I’ve tried removing the unit’s cover and letting it run, just to see if that was somehow creating a vibration or airflow issue — no dice. I have our guy coming to take a look this week but, like you, I’d like to have some idea of what the noise could be as I find that helps troubleshoot with more accuracy. Would love to hear what you came up with!