My car is an 08 with out mods, only 800 miles. When its idling there is a noise which sounds somewhere between a tap and a knock. Its not constant youhear tap tap tap then nothing and then about 30seconds later it will do itagian. Alsoit sounds louder underneath the car,you can hardly hear inside the car with the windows up. Once it's above idleyou can't hear anymore. It does seem to bein time with the RPM's. I can hear the injectors and it's not the same noise any ideas before I bring it back in??

Thanks for the help.

Todd

 

Could be your valve lifters. My dad's F-150 had that problem for the first 3000 miles or so, it went away with the first oil change. The reason why the noise goes away and comes back is most likely that when fans turn on the engine slows down a little bit which decreases oil pressure and which therefore results in causing that noise due to poor lubrication. Once the fans turn off the engine speed gets back to normal, pressure gets back up and the noise goes away.
P.S. if you take it to a dealership most likely they'll take a week or so replacing your oil pump and give you the car back with the same problem