2004 Mazda 3 - clank when shifting into reverse

Hi All,

Every morning I start my car and when I shift it into reverse there is a loud clank from the engine area.

Every time, after the car has sat for awhile, it clanks when shifting into reverse. Only sometimes does it clank when shifting into drive.

I brought it into the mechanic thinking it was something to do with the transmission. He said the transmission is fine and that the noise is coming from the "drivetrain spacer" and it'll cost $500 to fix.

The mechanic said that fixing it is optional, which surprised me because I thought he'd love to take my money. :)

So, here are my questions. What does this spacer actually do? And, should I get it fixed? Will something wear out faster if I don't fix it? Will it affect performance, fuel efficiency, safety, etc?


thx.
-Trev