Here is a copy of the reply I made to another question about the squealing noise. This applies if your problem is not a loose belt: I had just this problem on my 1995 Nissan Presea CT2 1800cc. I thought it was just a belt issue, but in fact insufficient lubrication to one camshaft was causing serious wear on the cam lobes, hence the noise. According to the mechanic this problem is very common on several Nissan models, i.E. It is a design flaw in the cam lubrication system. I needed a new camshaft, lifters, engine flushed, and replacement with semi-synthetic oil.
It is a 5 minute job to remove the valve cover and check the camshaft. I strongly recommend that you do this immediately.