Whats that duct piece called that has rubber, that you can put between a trunk?

Disclosure: I am a homeowner, not a pro... know just enough to be dangerous.

I recently put in a split system for the first floor... added 2 zone dampers, and now running a 16X8 to the second floor.

I've heard the hardest part of building a bridge is meeting in the middle... Apparently this is the hardest part of running a duct. I cut the holes, and my trunk running down from the upstairs is about 4 inches off from the piece running up from the downstairs. I only need to offset about 4 inches one way, and perhaps 2 inches the other.

I noticed for my downstairs, that when the unit cycles on/off, it makes a noticeable bang, so I thought for the second floor, it would be good to isolate the trunk using some kind of rubber trunk for a spell.

I am thinking I can kill two birds with one stone... Use that rubber piece of 16X8 to both reduce the noise for the upstairs (so when it turns off/on it does not wake anyone in the middle of the night) AND I can cheat on getting my trunks connected without heading back to the sheet metal fabricator.

So the question is... what are those rubber sections of duct called? I described what I want to the guy at the counter at the supply place, and he said hes never seen/heard of anything like this. But I know I have seen these... am I crazy? Or did I just get the new guy? Maybe if I know the right words, they will find it.

Thanks!

submitted by /u/bcjgreen
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