Inducer motor hitting metal.noise

Sounds like a bad motor bearing / bushing or a loose fan. Depending upon the furnace it shouldn't be too hard to check all of those things.

It seems the inducer motor would create a low pressure inside the heat exchanger.  If so, wouldn't the airflow thru the gap be from outside the furnace to inside the heat exchanger?

Regardless, I suspect your inducer motor is about to take a s*it.  I'd check for bad bearings and/or loose blower wheel on shaft if possible.  FWIW, I've never had an inducer last more than 8-10 years and they have all been fused together so hardly worth the effort to repair.

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... :)

No issue other than that it's failing that is. Not sure what he means by when the RPM is high. They are fixed speed, AFAIK, unless it's a two stage furnace, then it may have two speeds. If it's varying RPM, maybe it's because it's binding up. But I agree, I don't see any gas leakage issue, it's all fresh air through the blower. I'd probably look around and get a replacement to have on hand.

You're right, my mistake. So there is a potential issue with flue gas leakage if the blower shaft isn't sealed.

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.

The inducer fan on my Tempstar started "chirping" while still under warranty - but due to my dealer having gone TU, and local dealers niot being able to get parts in a timely manner, I did the troubleshooting and repair myself. The "web" of the squirrelcage was "fretting" in the hub (indicated by fine rust particals showing at the joint) so I brazed the web to the hub and it's been quiet now for about 12 years.

Does any furnace with an inducer have a chimney? Thought they were all direct vent.

You thought wrong.

Correct - my "medium efficiency" furnace is a non-condensing furnace, so uses a chimney - yet also has an inducer fan. (An inducer fan is used on ANY furnace with an "enclosed" burner)

There's no gas in the heat exchanger, you boob.