Engine 2005 4.6 horrible noise idleing with ac on only in gear (auto)


New Member
May 4, 2019
weed nm
just bought 2005 gt 4.6 auto
cold ac
runs great
idles great
after driving it home from purchase stopped in the drive, ac on, in drive. after about 5 seconds a horrible hammering started and got worse by the second.
engine also, as the sound got worse, started to stumble and rpm dropped.

i put it in neutral and revved it up and the noise went away totally.

ac was cold the whole time.

killed it. did visual check under hood. all was well.

cranked it, ac on, in park. idled 2-3 minutes, no noise at all.

put it in drive and 5 seconds later, horrible noise.

you can idle till the cows come home in neutral or park. no noise.

drove back to town and back. almost home was in 4th at about 1200 rpm and i heard the noise starting. it is very distinctive. pulled back into 3rd (about 1500 rpm) noise quit.

anyone else ever had this kind of problem?

the car has 92k on it. runs perfectly, ac blows COLD. trans shifts perfectly.

i love the car and will spend whatever it takes to fix it.
thanks for any help

ps: showing the cam position sensor code on my code reader.

owner just put new alternator on it last week
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