01 S80 T6 intermittent idle noise
 

My 01 T6 has an intermittent loud clicking almost thumping noise from the passenger rear side of the engine compartment. It's actually louder if you get down next to the car like it's in the exhaust. I do know what lifter and internal engine noise sounds like and this is not in the engine. The speed of the noise fluctuates but not with engine speed. Once you get on the gas it goes away. It only makes the noise when the vehicle is warmed up. It has 215, 000 now, but I've only had it since 210k. The day after I bought it it blew the cam seals out of the head thanks to plugged crankcase vent. On a positive note at least I know the timing belt and everything related as new now. The noise was there before the timing belt, pcv and oil leak repair. I'm not sure if it's related but it also has the surge on Accel that I've read about. Mine only does it when I first start the vehicle and accelerate slowly. When I had everything apart to fix the oil leaks and PCV, I did clean the throttle body. Lately I haven't noticed a significant drop in fuel economy. Typically averaging around 20-21, now it's down adround 14 -15. Again I'm not sure if that's related but any info helps. I am a Master Tech myself but I'm just now learning how to speak Volvo. I guess I'm a glutton for punishment. Is there some kind of valve or butterfly in the exhaust? Thanks in advance

usually when there's a big drop in mileage, you'd suspect a mixture issue (like an O2 sensor gone bad). As to the noise, there's not much engine wise - I'd think CV joint/motor mount (torque rods) for thumps and clicks. Does the sound go away if you shut off the AC? I'd also give a look and listen for exhaust leaks, possibly a rusted/broken hanger. What I do to get a better sense of what parts are in play is to look at a dealer parts web site for diagrams (try Tascaparts.com for the best prices) and mail order shops like FCP Euro and EEuroparts (all are east coast but there's good west coast suppliers as well) to see what parts are listed as the more common repair items. Also for the mileage, the electronic throttle mechanims are known to act up due to worn contacts and gunk. Given you had to redo the PCV recently, cleaning the MAF and throttle body can only help.

It makes the noise sitting still AC on or off. It's completely random and intermittent. All of the exhaust Hardware and piping looks good. And it changes Speed without the engine changing speed. I would think any engine accessory or even exhaust leak would only would change with engine speed. I'm leaving in it now if it acts up I'll record and send to you

I've been thinking about this one since last night. I'm wondering if the VVT camshaft actuator (what the belt rides on) is worn out and slapping around.