Englander nc-30 blower rattling

Mine had a bad bearing from day one and I screwed up and didn't get it replaced because I kept trying to convince myself it was vibration. It finally went south and I just blew it off because I ain't pulling up that liner and lugging that sucker out of the fireplace. I knew it was a potential issue when I put the stove in the fireplace and try to warn people here of it. That is why inserts have the blower mounted on the front.

I now use two seven dollar "personal fans" blowing under it on each side back into the fireplace and will never replace the thing. They do as good a job and they are quiet.

In your case it might just be vibration. Even back in a tight fireplace, can't get tighter than mine, you can reach the screws to tighten them. Failing that, do what car manufacturer's do when they find a rattle too late to fix it. Lay a magnet on it to absorb the vibration.