Tapping noise
 

Hi everyone. There is a mistery that I'm trying to solve.
1998 Astro van with 4.3 engine.
It runs fine cold, but starts knocking (high pitch metal to metal like tapping) when it gets hot. It knocks only when idles, stops when you rev it up until reaches the RPMs and then knocks again when I let gas pedal go. Also there are short bursts of noise when transmission shifts. Feels like it is fine when engine is loaded, and knocks when it idles or slowes down. also I can hear it when going at a steady speed 50-60mph on a flat road. It sounds pretty much like a lifter noise, except lifters were replaced a couple of months ago. Oil pressure is in a middle.
Any thoughts?
Thanks, all.

What grade of fuel are you using? Jim

regular unleaded

The valve springs are known to get weak with time in these engines. That could be contributing to your problem.
I would say it is a misfire that goes away at speed.

Bump up to Premium for two weeks, and see.

The last touched theory I think come into play here.Also the reason why the lifters where replaced is a important factor or maybe the reason why not they should have been replaced.These engines are 0 valve lash and a roller cam.It is highly unlikely it would be a worn lobe problem.Take off the dog house and let it warm up to where it is knocking.Take a really long screw driver and sound it out to location which side or if it is both sides the knock is coming from.To sound it out you put the screw driver at different places om the engine and the handle of the screw driver on you ear.Now I admit it does take somewhat of a trained ear,but you should be able to see what kind of a knock it is and try afterwards to describe it is the post.
If you I.D. it to be what seems to be a rocker arm ticking,pull that side rocker cover and tell me what you found.Leave open what you think it is until you found the problem.Don't let yourself talk yourself into a answer until then.

These engines are designed to run on 87 gas.If you found by changing that,I suggest you are just creating a symptom and not fixing the problem.I would think you want to fix it.The only thing that "could" create a ping like that would might be a crank sensor,bad ECM,or dist.

You never said if it was showing any codes either.It might be storing a code.You should have it scanned.

You need to tell me how many miles are on the van as well.It helps.

Take the advise above and let me know what you found.Need more info from you before I can help you more.

Thank you all,
The lifters were replaced because they were shut, because the coolant was leaking on them, and they were collapsed when I pulled them out. But this tapping noise was there besides the lifter noise. Also it has a lifter frequency. But lifter noise was there all the time, while this is only when hot and on the down transition from high RPMs to low, and on idle. It is not a detonation knock for sure. the engine has 230000km on it. No codes in the memory, no light ever came on. I was going to do what ASV advised, pull the dog house and locate the noise with a stethoscope, will do it this weekend.

Intake manifold leaks are somewhat common on these vans.Is that how it was leaking??.How bad was the oil??. Real mikey??.How did it overheat real bad??.

The oil wasn't bad at all, engine never overheated. Coolant leakage was very slow. I was adding about a quart every oil change.