I have a similar problem I hope you can help me with.

There is a tiny bit of play. It turns smoothly with a small resistance. The tensioner oscillates erratically at low speeds and there'* a corresponding noise that you might call a "rattle". Both the "rattle" and the oscillation goes away as the engine speed increases.

Moved to it'* own thread.

Sounds like you have a coupler going out. There is a write up on that in tech info/maintance I believe.

I'm really curious to know how backlash in the coupler would cause the tensioner to oscillate at low speeds. The oscillation must be due to variations in belt tension but how does the worn coupler cause that? (Note that I tried replacing the tensioner with no effect and the other two idlers spin free with no rough spots.)

It'* the slop in the coupler that does it. Take the belt off and you will likely find play when you turn the pulley back and forth.

What causes the slop to switch directions so as to cause an instantaneous change in belt tension? It would be this change in tension that would cause the tensioner to oscillate erratically.

The rotors are solid steel and heavy. Changes in air flow, changes in RPM, all cause movement at the coupler. I've seen several cars with a jumping tension and found slop in the coupler. I changed the coupler and the jumping went away.

That'* very interesting. I was told that it was likely a bearing but I found no other evidence of that which is why I'm going through this exercise. 5-degrees of slop seems awfully small. Do you suppose that so little could still make the tensioner bounce as well as make the noise?

If it sounds like it'* full or rocks when running, an dif you remove the belt, feel slop, but still feels smooth when turning, the snout bearings should be good. however, the needle bearings is another story. For that you need to remove the supercharger and inspect the rotors. If the coating is still there, the needle bearings are good, but if your missing a lot of the coating the needle bearings are bad and need to be replaced, or just swap out the supercharger all together. I have 3 sitting on my shelf if your in need of one.

Since it'* apparently so cheap and easy I'm going to try a new coupler first. Thanks for you input.

I did a write up on how to do that, it'* in tech info/maint I believe. If you have any questions or run in to trouble, just post it.