2002 WRX, 5MT

I had a transmission rebuild about 2 years ago where they replaced the front and center differentials (which the dealer said were worn due to a misalignment issue, possibly introduced when the rear diff was replaced at 80,000 miles) and they filled the transmission up with full synthetic, something I noticed when I checked the dipstick about 6 months later. I'm getting close to the time (35k miles) when I might replace the transmission fluid, but the full synthetic, while it seems to work fine, is NOISY. At low revs I have a whine that I never had with the factory fluid. Any issues with switching back to Subaru factory fluid? Do I need a "flush" in between, something that has always seemed to be a mechanic's favorite expensive placebo to me?