L322 Gearbox squeal when cold

Hi all, i had my gearbox oil changed earlier this year at 100k after I was starting to get the notorious cattle grid noise from it at certain speeds. Filter etc was also changed and the oil was totally drained from the cooler too. It was refilled with the correct oil, esso lt7741 or whatever it is called.

Since the change, the gearbox makes a squeal and a light swooshing sound when driving from cold. It goes away after 5 mins as it warms up but it very annoying.

I assumed it hadn't been filled to the brim but it's just come back from the garage where I asked for it to be topped up and they said it was already full. Gearbox was cold when they opened the fill plug but oil actually came out when they unscrewed it so they just closed it straight back up.

Any idea what this could be? It was silent when cold prior to the change.

Gearbox oil has to be topped up with the car horizontal, the filler hole is your level, so when refilling as soon as oil starts to flow from the level hole that’s it no more oil in. The car must be completely horizontal or you will put much in or not enough. To much oil and gearbox pump will struggle. Incorrect oil I.E. to thick oil pump will struggle. Oil thickens when cold.

Engine must be running, oil temp at around 40degC, go through the gear selections pausing in each briefly, then back to park, remove filler/level plug, thin bead should dribble out at correct level.

Yep, the car was completely level when checked and when originally changed the correct procedure was completed with changing gears etc. I can't understand why its doing it. The noise is a high pitched whirring noise which increases as the revs build. Sounds like a poorly maintained taxi! Any other ideas?

Only thing I can think of is the pump is cavitating due to fill level....hmmmmm

Was it a megaflush or similar they did? .... This uses a machine to drain and refill the box.

I actually gave the mechanic a guide from either this website or one of the other forums when it was done. It was done by just undoing the drain plug but then a hose was disconnected or something while refilled to drain what was currently in the cooler or transfer box to push out whatever was remaining in the system. It took almost 10 litres if I remember correctly. I can dig it out if needs be. It really sounds like it is just low on oil so I was amazed when he said its full to the brim.

Gearbox oil is a really good detergent and cleaner.....

In the box are all sorts of hidden nocks and crannies were crud and debris can hide harmlessly out of the way.

If almost 100% clean fluid is introduced, this can have the effect of strip cleaning the insides and loosen all this harmless crud into the fluid, which can cause premature wear to components...this is known as 'New Fluid Syndrome' which is why I asked if it was a 'Megaflush'...as this completely empties the system and replaces it with fresh oil....

Draining the whole system is similar, which is why most advocate just emptying the sump, then filling up again (6-7 litres or so)...then after a few hundred miles, do another sump-dump....the first changes about 66% of the oil...the second will change another 66%...so the amount of new to old oil ratio changes to about 85% new fluid....and then finally do another sump-dump after another couple of months....66% changed again....so the ration now is about 97% new oil.....

It prevents a sudden loosening of crud and anythat does work loose is removed onthe next fluid change and so on.

I'd re-check the level again....and ensure it is done to the letter of RAVE....at 40degC, engine running, through gears and check....also check the look of the oil...if it is starting to look grungy already, it could be loosening crud inside the box.

Ok thanks Saint, I'll go back to my mechanic and advise. He did say the newish oil was already quite dirty but was adament it shouldn't be making any noise from not being full. Thanks again.