Loud hissing noise

After my friend did a minor tune up, new spark plugs,ignition coils and oil. changed because he got a misfire code. First the check engine light just went off than drove maybe forty miles and check engine light came back on flashing especially during acceleration. now upon start it screams like a ready teapot with acceleration. the screaming subsides or goes completely away. The engine runs strong no skips nothing but this is very annoying. Where do we look to eliminate this noise? It is so loud in the engine compartment at an idle it is hard to isolate where it is coming from.