Loud Engine?

Since i bought my k11 iv noticed the engine being a lil loud,until when i checked the oil i noticed oil was below the mark..so today went and got my service parts,10/40 oil,oil filter,spark plugs..changed the lot started up the car and let it run for a while and noticed it was still loud on idle,but when revving it or driving its fine,any idea what it can be?
 
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