Crazy. I am having the EXACT same problem in my 91 Terrano. It is only happening while moving. It also appears to be coming from the front passenger side (driver side for most of you, front left). It happens during acceleration, while turning, and during bumpy driving. I can induce the problem while driving by shaking the wheel left to right at around 40kph to 50kph. I am thinking my problem is the belt or the power steering. When the engine is off, and I pull and push on the belt, it makes some moaning sounds, so if that were sped up to about 2500 rpm, it could be it.
I will change the belt, and do some lubing and post back on if it changed. If not, then I don't know what to do.