05 G35 Engine Knock

Replaced spark plugs about 10k ago and a bad cam sensor. 159K miles on my 05 G35. MPG around town is 18.3, I was getting 20 MPG. The vehicle runs great but I do get a knock at idle, it's almost like getting a bumper tap. No codes. Any ideas on what to check? I've always ran 83 octane and never had a problem. :confused: