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pepepeter, you exchanged dirty hypoid with synthetic clean hypoid and thus have removed grindings in the form of small metal particles. These particles grind up to even smaller particles and with time and use and blending with hypoid oils become a grease of sorts and lubricate the crown/pinion and bearing surfaces.
The removal of the grease type grindings may have left a space where frictional parts could cause this minor squeal. You will have to drive more and let the new hypoid oils get into all the areas.
After a month it should be quiet but I suggest yet another exchange as any particles left in suspension can now be drained then replace the hypoid every 2 years. The use of synthetics in crown/pinion are not required because standard hypoid does a great job but remove it every 2 years.
Leaving oils of any kind in service for too long can create a problem with bearings, rubbing surfaces as cooling is diminished, lubrication suffers from crap build up in the oils and O2 oxygenates the lube plus dirt and rust from ferrous and non ferrous as copper, brass get into the mix and add particles to impede lubrication.
Just look at the mess that came out of your gear box and that box is enclosed. Be very watchful of the used oils coming out because they indicate wear.
The Rav4 uses a front differential mated with the transmission and the front diff is lubricated with ATF. The front diff is 100% a work horse in the drive line and crown/pinion gears present a great shear force. ATF compared to hypoid in this extreme condition can be a disaster. The saving grace is the ATF is being pumped around the front end and the differential. The oil ATF is a WS, synthetic but in my opinion should be checked , tested and replaced sooner than what is expressed in the manual
Differentials and lubrication.
I own and operate a 33 year old Goldwing. Rear end takes 80W90 hypoid. Put in 90W140, because the only available grade. Within a week the oil was migrating up and out the breather. Dumped it and ordered the correct grade. Be very careful with oil grades to match your ambients and machine temperatures