Your noise sounds exactly like mine does, it does it over speed bumps. But it also does it sometimes on full lock accelerating and braking in first gear, I had to find an empty car park to try this though, it doesn't do it on normal driving.
Unfortunately I never tried that, and I can't now that the noise has seemingly gone.
I'm pretty sure the IS300 uses the same size of ARB as the IS200, so would have the same bushes.
I wasn't exaggerating about the ARBs, I still think they're absolutely awesome and I highly recommend them ! :D