Lennox Fan Noise

I have a Lennox Merit series natural gas forced-air furnace, 5 years old. It just started making a very loud rattling/vibration noise during the startup/warmup cycle when just the exhaust fan is on (before the circulation blower fan kicks on). I manually checked the exhaust fan by turning it and pulling on it to see if there is any noise/friction or play in the bearings that could be causing the noise, but it spins effortlessy, no signs of friction or play in the bearings. Also, the vibration noise is so loud that I doubt this small fan could cause it, and like I said it's more of a vibration in the sheet metal than a grinding noise.

Is there a panel or baffle of some sort in there that could have come lose and cause this noise, and if so how do I gain access to repair it? Thanks so much in advance for the help!!

Greetings Clayton welcome to the Forum.

Some/many times, fans and Cages have a Balance Clip stuck on them, for Balancing.
It could be that it has come off, and seemingly be in good shape (no looseness) and the vibration occurs when the fan speeds up.

As far as accessing the Unit, look for any removable panels which would give you access to the area.

I have used Silicone Sealer on joints where metal touch another piece, sorta glues them together a little better, and dampens any vibration to some extent.

Have a good Day !
S.W.