just fitted a bbost gauge to my 93 wrx and noticed the needle vibrates and makes a buz noise .is this normal or have i make a mistake anywhere

Think this is normal under hard acceleration. I think some people have fitted a restrictor inline with the hose to the gauge to stop this.

Mark
Mine did exactly the same, especially when engine was cold. The cure is to put a small clamp on the flexible pipe, and adjust until the needle is smooth, but still reacts quickly to throttle changes. The problem is finding a small enough clamp! If you use a fairly flexible connecting tube you may get away with using a tie-wrap, gradually tightening it until the needle smoothes out.
Cheers JohnD