Msecrets18

Hello. Can anyone tell me what the green cap thing is on the end of tje black hose? Mine is missing and my 2006 pontiac g6 has a knocking sound at high rpm. Would this cause this? Thanks

Hello. I just bought a 2006 pontiac g6. I find that at high rpm it has a knocking noise and the oil looks like its never been changed its so thin etc. Can dirty oil be the culprit or is this knocking mean my engine is shot?? Its a v6 with 125000 miles on it and is in beautiful condition otherwise. I cant afford a shop so what?/how can i check myself to diagnose this?? Thanks everyone.