Squeaky sound....???

[Dr. Jerryrigger]

So my girlfriends 89 Toyota Cressida is making this squeaking sound when it drives over bumps, coming from the front wheels (it's rear wheel drive). It sounds like a dry ball joint but a higher pitch (though I've only heard that on a small truck, so it might sound the same for this car).
The sound had no relation to turns, just bumps in the road. It only seems to happen when going over 25mph. I cant reproduce it by jumping on bumper while parked. It seems to be happening independently on both sides i.e. pothole on left make sound on left wheel, pothole on right make sound from right wheel.

So I assumed it was a dry ball joint, and popped a little hole in the boot, so I could spray some white lithium grease in it (only one one side of car). Oil squirted out, so it was clearly not dry, adding the thinner grease didn't make the problem go away either. So now I really should replace that boot I put a hole in... But I still don't know what could be making this sound... and how dangerous it may be.