I'm having a weird issue that I haven't seen before. I have a 2005 S60 T5 with about 98k miles on it. What is going on is whenever the engine revs up over about 2k RPMs, it makes a clicking noise over the 5th cylinder. It does not increase in speed as the engine does. I have no check engine light or pending codes. It sounds like it is over the 5th cylinder (definitely not a knock). I pulled the spark plugs and it looks like the engine is running pretty lean in all cylinders but the plugs look alright. Unfortunately, I don't have access to a scan tool or gauges, just a code reader and basic tools. The noise also eventually goes away when the engine speed increases over about 3.5k RPMs. Anyone have any idea what could be causing this? I've just never seen something like this before.

 

Did you ever figure out what that clicking was? I have a very similar problem with my xc90