Oil pressure light comes on at 5-7psi. If you are getting the light on at any time after start-up you have MAJOR problems. Low oil pressure reported - quantify: 20psi 30psi? Shop talk is cheap - data is gold. I cannot give a qualified answer on exactly what pressure is 'normal' as I do not have a manual covering that engine, so you will need to research that. Reports of low oil pressure and 'noise' is never a good sign. Can you quantify oil pressure independently (hire/borrow a gauge)?
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL (X 2 - 1 White and 1 Light Blue); E34 535i; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i