X300 3.2 AJ16 strange noise

[a8hex]

Half way around Reading this morning my XJ6 started making a very odd noise, it varies with throttle opening and engine speed, but the strange thing is that one more than about 25% throttle opening it goes away and sound quite normal, back off and the noise is back, even at motorway speeds. Inside the car it's like a loud angry wasp, outside while driving it didn't seem to cause people to look up or stop and stare, but on opening the bonnet and listening is sound decidedly mechanical and doesn't appear to be more noticeable on from one particular side, so scarily sounds like it could be coming from the main engine assembly. Given the sensitivity to the throttle I'd assumed it was something inlet side, but doesn't sound like that.
So any idea what might be varying with the throttle opening?

[SpeckledJim]

Top or bottom and front or back? And is it the same in gear as out of gear?

[a8hex]

SpeckledJim said:

Top or bottom and front or back? And is it the same in gear as out of gear?

Top I think, seemed fairly central, doesn't change with the gears or speed. The frequency changes with the engine speed and the noise goes away when the throttle opens up, it also stopped at one point when my foot was off the throttle but then it came back again.
Is the valve train affected by throttle openings? I know there is variable valve timing on the AJ16, but I assumed that would be a function of the revs. no idea where I got that notion from

Edited by a8hex on Sunday 28th December 22:30

[Cerberaherts]

If it appears more under load it could be a timing chain tensioner....

[a8hex]

That's the weird thing. Under load she's her normal serene smooth self, it's when you back off or cruise she gets noisy.

[a8hex]

OK, I had another look today.
It's the air conditioning system.
In all the years I've owned her I probably only ever driven with the AC switched off when it's been for an MOT or something where they always switch it off. So I hadn't thought to try this the other day. I guess when you put your foot down they cut the AC pump to allow more power to be sent to the wheels.