Have a knock that comes and goes...don't hear it usally unless I've been idleing for a while...example. Today I get out of my truck and leave it running to go in store, walk out the store and start talkin to someone for about 20 mins..its knocked for less then 6 seconds then went away...countinued talkin and 5 mins later it started more constant, rev'd it up and it went away. I then left and drove less the a mile and park and sat there a few mins and nothing...it comes and goes...was thinkin maybe a lifter, but seems pretty loud to be a lifter..