Odd noise, rev associated.

[Davidqhog]

Morning All,

Yes, the dreaded "odd noise from the engine area" post.

My Super V8, 2005, with > 250k miles recently had the gearbox rebuilt. The TC was slipping badly so a full rebuild was commissioned. Got it back and it's super smooth, changes quietly & smoothly, etc. All I could hear was the thrubb of the cylinders firing and some wind noise from the mirrors. Bliss.

After a couple of weeks I started hearing a noise from inside the cabin. It is associated with the engine revs so goes up as revs increase then drops as the box changes up. It does it at idle in Park too so it's not the transmission.

The noise is like something with a bearing that's on its way out. Alternator seems like a good fit but the local garage say that's in good shape. The engineer could hear it but even with a stethoscope couldn't pinpoint it. Can't hear it outside the car as there's too much other noise. I've been advised to drive it until it gets louder!!

Anyone have any thoughts on what might be the culprit?

Regards,

DQ

[BrianG]

The air conditioning compressor bearings is a candidate. Mine, at 144k miles, is making a grumbling noise and varies with engine rpm (stops if you turn the AC off) so it's on borrowed time. The noise is quite audible in the car but can't be heard outside. Other candidates include the serpentine belt tensioner idler pulley bearing and water pump bearing. As your car is supercharged you also have the added delights of the supercharger bearings and as your car is high mileage, has the supercharger ever had the oil changed inside?

[Davidqhog]

Thanks for the thoughts, Brian.

Supercharger oil was changed about 6k miles ago. Possibly the first change, I don't know. I'll try turning th A/C off next time the days are cool enough!

Regards,

DQ

[BrianG]

David,
An update on my aircon compressor noise: it might be worth getting your system gas pressure checked if it's not been done in, say, the last two years. Here's why........

This week I finally got fed up with the compressor noise and aircon getting worse so had the car booked in for a check by my local aircon specialist, expecting a failed compressor. Turns out that by now there was NO gas to speak of, and the system was almost at atmospheric pressure. He'd previously serviced it in August 2022 but it needed a partial recharge in August 2024 to be fully working (two years is fairly average apparently), so after such a short time time I was therefore expecting a big leak somewhere.

It was fully pressure tested with nitrogen at 5 Bar over 30 mins and he was surprised it had lost no pressure, so he went looking for small leaks with soapy bubbles spray. There are two largish aluminium tubes that stick up on the left side of the engine bay against the bulkhead, the tops covered by black plastic dust caps. These are the system filling and test ports and each have a Schrader valve under the dust cap. The high pressure filling port on mine was blowing nice soap bubbles so he replaced both Schrader valves to make sure and then refilled with fresh refrigerant gas. The compressor is now totally quiet with super cool aircon, so it's not on its last legs after all!

[Davidqhog]

Hi Brian,

Thanks for the update.

I turned off the A/C yesterday - not such a hot day - and the noise stopped. Did it a few more times and it's defintitely the A/C.

I'll live with it for a while and when my Daimler is back on the road, I'll get the local garage to perform your tests.

Thanks very much, old chum!

Regards,

David Q