F21 grumbling noise under throttle turning right.

[D4MJT]

This will be a bit of a lengthy post so apologies and bare with. I'm running out of ideas trying to troubleshoot a problem I'm having so I'd welcome any suggestions however daft they may be, I'm sort of at the point where I've been looking at it that long I can't see the wood for the trees.

As concisely as I can:

I've got an F21 M140i which had the ZF8HP50 in it. I've recently swapped in a ZF8HP70 from a 330d which will take more torque than the previous box. It's a recognised swap / upgrade that's been done before, I've driven another car with the swap and it's brilliant. There's xHP gearbox software specifically for the HP70 in the M140i chassis so there is no problem with the actual swap.

Immediately after the swap, I got a really strange noise under very specific conditions. Under throttle, whilst turning right, I get a grinding / grumbling noise that sounds like a sort of mix between tyre noise and a completely knackered wheel bearing. It is not present whilst driving straight not does it appear to be present whilst turning left, this is what's really baffling me.

Since then, I've had the box back out and also carried out all of the following:

New torque converter
New propshaft guibo donuts
Diff Service
Gearbox Service
Checked all subframe bolts despite them not being removed during the swap
swapped the front wheels side to side to rule out a tyre / wheel issue
removed previously fitted wheel spacers
Checked downpipe brackets and exhaust to rule out knocking.

None of this has made a slight bit of difference. In desperation I've now taken the HP70 out and fitted another HP70 unit from a known low mileage car.

It's making exactly the same noise. The HP70 shifts absolutely perfectly and selects all gears fine. The clutch values and chargetimes are well within good limits. The TCU for the gearbox is reporting no errors at all.

I'm going to change out the front left wheel bearing out of desperation as it's all I can think of at the moment, but it feels fine rotating the wheel and has no play and I'd expect the noise to there all the time really.

Any ideas very welcome however daft.

To add: The noise is engine speed rather than road speed dependent, so if you're driving round a roundabout in 5th at 30mph, the noise is more subtle than if you're driving round the roundabout at 30mph in 2nd. So to my mind it can't be propshaft related or wheel bearing related really.

Edited by D4MJT on Monday 31st May 17:35

[D4MJT]

I resolved this, it turned out that the HP70 box is very slightly longer, and with the torque from the engine can twist on it's own mounts just enough so that the back left hand side of the box just lightly touched the actual gearbox crossmember. It's very hard to track down as the box doesn't move enough to replicate it stationery unless you put a torque wrench on the rear mount bolts and whack some force through it, then it moves just enough to touch it.

Shaved 4mm off the gearbox crossmember with a flap wheel and the job is a good'un as they say.