What is this on the Ductwork?

Does anyone have any idea what these black stickers are on the ductwork? They felt slightly tacky to the touch. I thought they may have been covering holes, but they were not. They were on all of the ductwork, and I have never seen this before. Any ideas would be appreciated. Thanks in advance.

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Maybe cross breaking wasn’t enough and the are to help keep the expansion/contaction noise down. Never seen them before really…

Larry’s got it.

It damps out the duct pop on blower start up and shut down.

http://www.acousticalsurfaces.com/hvac/duct_vmax.htm?d=26

Never seen it either, but that is what came to mind first too.

Possibly a thinner sheetmetal than normally used and the duct was drumming with the velocity of the air. :slight_smile:

Thanks guys. Interesting, I had never seen this set up before. Looks like a good product. Mike, how do you always seem to come up with the magic google location?

Is that cheaper than dyna mat?:smiley: