Odd Problem

My mom has a 2001 Beetle and my it was running ok, then i looked under the hood and it had no oil, like none, so i added some gtx high milege the right grade, 5w-30, and i also changed the air filter it was nasty, i put in a new k&n filter and it was running great until today i started it up and the engine or the exhaust i think was makeing a deep sound, like those teenage cars, deeper than normal. Any suggestions, thanks a lot guys :). Ps- the air filter box wont close all the way, dunno why is that it.

The noise is probably from the air filter housing if it's not closed properly.

A VW mechanic told me never use any air filters other than genuine VW!

Thanks A lot guys, oh and i look at the filter box, its not closed all the way, you were right, gotta figure out why its not closing though, any thanks for the help.