Is it a squeal or a whistle, If it is similar to SR20 could be cam lobe oiler's, its very common in earlier model SR20's (as said) and they squeal ( pull rocker / cam cover off and you can check if they are blocked or there is black oily gunk on / in them)
The other Issue I had that I couldn't figure out was my car again SR20de would sometimes whistle / squeal while at idle (mostly when sitting in traffic when hot i.e. motorways etc), and sometimes only when you turn the engine off it would let off a small squeal / whistle sound, finally figured out that water pump was on its way out. (but I would presume that would have been done when belts were done)
I know its not the same engine but could be possibly similar.