[jhallenworld]

The usual failure mode is that it sounds like a loud vacuum cleaner for ~90 seconds after cold start up.. not sure about clunking.

[Alupis]

I owned an RX-8 which had an air pump (secondary air injection) which would come on for about 30 seconds on a cold start. When the pump started going out, it would make a metal grinding sound. Drove like that for a few years until it quit working, and didn't think anything of it until SMOG time - $600 for an air pump which works for ~30 seconds...

[heywire]

Exactly. Cheverolet Trailblazers are well known for this (I owned one and had to replace said air injector)