I came home today to find that the inside part of the furnace/central air is making a loud knocking noise. It seems to be working fine, but is very loud. Any ideas? Thanks

The only thing inside that moves is the blower. It may be loose. May just need a few turns of a wrench.

Sorry I messed answering yesterday. I spent most of the day at a big outing with 15-20 dogs, mostly Labs.

I also have a carrier A/C (five tons) for seven years and noticed the noise when the A/C started. I opened the cover and saw two weight adjustment clips on the fins of the blower and shifted them left and right a bit. I did notice a big difference in terms of the noise level. However, the noise is still there at a much lower level. It looks like the blower is hung with lots of sheet metal screws and is behind a small electrical control pannel not very accessible (all surrounded by a sheet metal case). Is there a "short cut" path to oil or check the blower for losen fasteners?

I am doing what I can to help, but do not have enough experience to help with details like that.