2002 A4 6 cyl

A while ago the engine picked up a clicking noise. Sounds like a collapsed lifter, rear cylinder, passenger side. Gave it a dose of Marvel Magic Oil with no improvement. Now it is starting to sound more like a clack than a click.

Went to the Audi dealer to get repair estimates and the service guy said in 10 years he has NEVER seen a collapsed lifter on this engine. Another possibility is a loose rocker I suppose. Before I pull the valve cover I was wondering if I could get some input on likely issues.

I think I read somewhere a collapsed lifter could cause a cam lobe problem - or maybe not. Also I heard that Audi recommends/insists ALL the lifters be replaced, at $60 per, if ANY lifter needs to be replaced.

Get me up to speed please.

Thanks.