Bicycle Mechanics - Using 9 speed rear deraileur on 8 speed everything else?

Ok so here's my situation. My 96 trek 1000 road bike makes rattling/clicking noise whenever I pedal front or backwards. I take it to the LBS and they say I need new chain, middle chain ring, and cassette. I went home and replaced all three of those, but it still makes a noise. The noise is less loud now but it's still there. I have located the noise to the rear derailleur. Sounds like whenever a link comes into contact with the pulleys it makes a slight sound. My rear derailleur is shimano sora 9spd. The cassette is 8 speed and so is the chain. Could it be that the 9 speed does not work with 8 speed? Please let me know what you think.

My commuter is 8 speed Sora except for 9 speed Tiagra RD. Everything is quiet. I'd suspect an alignment issue (bent hanger or bent RD) or adjustment.

Could it be that the 9 speed does not work with 8 speed?
No.

How's the shifting? If shifting became less reliable at the same time the noise began, then you might have bent your hanger as CACycling stated.

High mileage chains can get a bit noisy as the rollers and plates wear, though noise may be lessened with fresh lube. Of course, you replaced the chain and still have the noise, so....

Have you checked the pulley with the chain removed? Perhaps a bearing is the culprit.

It could possibly be your BB as well. Have you ruled that out?

99% sure it's not my BB. How can I tell if my RD hanger or RD is bent?
Shifting seems fine. I shift up and down the RD just fine.

From Sheldon Brown

You can roughly check alignment by eye, if you look at the derailer from the rear. The two pulleys should be lined up exactly underneath one another, so that the chain runs straight up from the tension pulley (http://www.sheldonbrown.com/gloss_ta-o.html#tensionpulley) to the jockey pulley (http://www.sheldonbrown.com/gloss_i-k.html#jockeypulley), with no sideways slant. Compare the line of the pulleys with that of the sprockets above them. (http://www.sheldonbrown.com/derailer-adjustment.html#rear)

Right Thanks. Today I got behiind the rear tire and checked and the chained lined up with both pulleys. The sound is now almost completely gone with some lube added. Maybe it needed some lubrication. Thanks everyone.