Hi, I think it could well be the roll bar bar bushes as the ride height for the bushes is set at the factory in the centre of where the up and down movement, arc occurs, by lowering the car you change the dynamics of the arc and if you consider that the bushes are clamped so the bar is gripped at the optimum point to allow the bush to flex within given movement parameters whilst still gripping the bar without moving on the bar, try giving the bush that side a liberal hosing down with WD40 or similar if it stops squeaking new bushes it is.
Steve.