Let's play a game of what's that sound?

The car has 99,269 miles, and it's the 4.2L Supercharged V8. I don't know what to call it. A knock? A bag of rocks? When you rev it, the sound disappears. It's making that sound only at idle. I'm suspecting the supercharger, but I'm not sure. The car is running great with no perceptible loss in performance. There are no codes or MIL.

Any thoughts?

 

It does sound like a light knock - is it too rhythmic to be supercharger?

You could pull both serpentines & run the motor briefly to check if it's anything in the accessory train.

 

Well, good news (probably) is that it doesn't get louder with RPM. So it's probably not a rod bearing. I would start doing the poor-man's stethoscope test and use a wooden dowel on the front top of the engine (supercharger coupler), and then at cam covers (timing chain tensioners). If it's way back at the rear of the engine, maybe a torque converter or flex plate rattle? but yeah, the stethoscope or dowel test will tell you a lot more. If it's a timing chain tensioner, shut it down and fix NOW!

 

I went ahead and removed the supercharger belt and spun it by hand.

I feel some play when spinning it and I can hear some noise as well.

Take a look and listen and tell me what you guys think? Is it normal or is it the likely culprit?

Also, I just now realized the car does not make that sound when the engine is cold.

 

run it with the belts off.

 

I had almost that exact sound recently on my 1998 XK8 - sounded like a rod going on the diesel truck that was beside me at the red traffic light ———- until he pulled away and the noise stayed grrrrr.
Parked in my garage and started snooping. It turned out that 2 of the 3 bolts on the power steering pump pulley had partially backed out - tightened them back up and the sound disappeared.
Horse Shoes in uncomfortable spot LOL

 

Well done. Sure hope it turns out to be that simple for Gian....

 

I sure hope so too. Unfortunately I haven’t had the time to keep looking into it because of a loss in the family and intense academic workload in the past few weeks.

 

Good suggestion.

I let the the engine run and warmup while taking a listen and looked at all the pulleys. Everything looked normal at first and there was no noise. After a while it started making the noise and I was able to see the power steering pump pulley shaking significantly.

I tried taking a video of it but I could not get my phone camera to focus on the pulley.

When the car cools I’m going to try tightening those bolts. I can only hope that solves it.

 

Looking at the pump and pulley for my model on rockauto and image searches, it seems to have one large hex internal hex bolt holding the pulley instead of the three bolts on your car. There’s a bearing on the shaft to which the pulley is pressed on. It very well may be that that bearing has gone bad.

I wonder if I can remove the pulley and change the bearing in-situ without having to drain the system and remove the pump to change the bearing on a bench? It seems accessible from underneath the car. But I guess pressing a bearing in-situ may be a tall order.

 

As suggested elsewhere, l think you might need to run it up to a temperature at which the noise begins and then remove drive belt and check again. Even if it is steering pump pulley issue you need to verify source prior to changing out bearings etc.

 

My guess would be the supercharger - nose bearings or coupler. Just remove the supercharger belt and let the engine run to hear if the sound has gone.

 

I finally got around to removing the belts.

Without the supercharger belt the noise got thinner but was still present. When I removed the serpentine belt, the noise disappeared.

After inspecting all the pulleys, it turns out the serpentine belt tensioner pulley has play. The supercharger snout pulley also has some play.

I seem to have both a failed serpentine belt tensioner pulley and a failing supercharger snout coupler.