is replacing the timing chain guide a necessity?
 

I've heard this is made of plastic and sometimes causes failure. Is it something most of you do?

The labor required to just get to the chain makes it almost cost prohibitive. That, unfortunately, is what broke and caused my 740iL engine to eat itself alive. $13,000 and a month later, I had a new and modified engine in my car.

BMW uses plastic in the engine bay like it's going out of style.

oh - it can't be that bad. On the 4.2L V8 in my S4 the chains are on the back of the motor. So guess what it takes to get to them? You guessed it - motor pull.

Typically this isn't a problem, but if you have a rattling at start-up that could be the chain guides and you will need to replace them. Generally it is a replace only when they fail kind of deal. And if you get to it soon it should not do major damage to the motor. Notice I say "should". Anything can happen with a mechanical system...

I love my S4. Powder coating my calipers black on monday, and adding the S4 logo clip.

Yeah, but does the Audi 4.2L have the same potential timing chain problems the BMW 4.4L have?

Don't forget, rattling at startup can also be a VANOS issue, which is a very common problem w/ the entire M62-S62 line, which is all basically the same block anyways.