On Sun, 31 Dec 2017 11:06:33 -0600, dpb wrote:

On 12/30/2017 6:14 PM, Raymond wrote:
I have a gas furnace with inducer motor. Recently it started to make a
noise
that I suspect is some kind of metal grinding noise. It only starts when
the
rpm is high. I tried to fix it by putting a piece of metal ring to the wall
space between the motor and heat exchanger and that solves the problem. But
I'm afraid of the negative pressure and gas leak from the gap. Is there any
better idea ?

I see no issue, but use gasket material or some sealant if you're concerned.

The one here vibrates against something occasionally but can't as yet
tell just what...it's tight against the mounting bushings so isn't
actually that interface. So far, putting a little pressure on it to
sorta' readjust keeps it quiet for a month or two so that'll probably be
the fix until it quits...

I fished a dead bird out of mine. It was making quite a racket.
A chimney cap solved that problem.
Why don't you take the vent pipe off and take a look inside the casing?
A chimney cap solved that problem.