I bought a 2010 Grand Caravan with 60k miles on it from a dealer and at about 70k I noticed it was using a lot of oil between changes, 2-3 quarts. Not too much later I started getting a ticking noise once the engine warmed up. I ran a bottle of flush through and drained some nasty oil and pulled the valve covers. The upper was now pretty clean and all the rockers and rods were solid, no movement. I filled it back up and ran some MMO through as well and it started running better but still ticking like a bad lifter. I pulled the heads and replaced the lifters and cleaned a bunch of carbon build up off of the tops of the pistons. I started it back up and after about a minute the ticking started up again. It's now at 80k and no more oil usage but the ticking/tapping continues. When it's cold it takes about a minute to start tapping and once it's hot it starts tapping immediately. I'm wondering if a bad oil pump can do this or if it's vacuum related. It fades out at idle sometimes and definitely follows RPM. Sounds like a lifter but definitely not a lifter? Any ideas?