Furnance vent making popping noises

Hello,
I'm looking for some advice on how to eliminate the popping noises (tin-canning)that are coming from my heating ducts. They only occur when the furnace is starting up and shutting down, and I know that the noises are due to the expansion and contraction of the metal, but the noise is excessive and the vent is right next to my bed.
The problems is complicated by the fact that my bedroom is on the 2nd floor of a 2-storey house, so I can't get to the vents to see where the popping occurs. I've tried to slightly shift the vents from the basement in the hopes that the noise will go away, but that didn't help.
Any ideas would be very much appreciated.
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Start in the basement. Watch and listen as the heater starts. You may not only hear the pop, but you may see the duct flex. Once you have narrowed down where it is happening, you may be able to add a stiffener or better mount for the duct.
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I did that already. The noise/flex is not coming from the basement, but rather the 5-6" diameter circular feeds just under the subfloor of the 2nd floor bedroom. If I pry the duct open at the vent opening, I can just get my hand into the subfloor opening and grad the metal tube, but I still can't tell where the flexing is occuring along that run. Would you suggest spraying some foam insulation around the metal tube (outside) to dampen the noise? Is there something else I can do?
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This is Turtle.
Rare does round duct ever make noise ! It is most always square ducts that does this. You probley have a square duct involved here in the walls making this noise that the round takes off from and the round duct is just bringing it to your rooms or wall .
Now when see what your saing here about duct shifting and snapping to make noise. Usely it is cause by too big of a temperature rise on the air of the furnace. You need to check it and make sure your not going over a 70 or +F rise in temp. between the air going in and the air coming out in each room. You may have to come back to a 30F to a 50F Temperature rise out of your furnace to stop this. You can check the Temperature rise your self with a thermometer and get the temperature of air coming out the closest vent in a room and then get the temperature of the air going into the return air grill and then subtract one from the other. This will give you your temperature rise and don't let it get over 70F Temp. Rise. I like a temp. of 30F to 50F temperature rise on my equipment.
If you wanted to discuss it by e-mail here. E-mail me and discuss it.
TURTLE
You will have to increase the blower speeds or decrease the gas pressure to lower this temperature rise and i might suggesat a hvac service company to lower it for you.
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