I have a my95 wrx wagon. the car seems like it is losing power/boost?

the problem has seemed like it started after the cluch was changed but this is not deffinate. the only thing i have noticed different is that when the car hits 3k rev there is an unusral noise comeing for the engine bay and also happens when you back off of the gas....

The noise lasts for around a second and is intermitent kind of works like a dump valve would the only way i can discribe the noise is that is sounds like a scalextric car going around the track. i have checked the boost pipes but cant find a leak?

Please help

Mike