Need help with an engine noise 400bb

[redgum78]

Hey guys, I have an engine noise that is driving me nuts trying to diagnose. Hoping for some help from the engine masters on here 

It's the original 400ci 2 barrel on my 73 Charger. 55,000 original miles. Stock as a rock other than an after market distributor and a double row timing chain set. The timing chain set was installed because the plastic/bakerlite coating on the original cam gear was coming apart.

It starts fine, runs nice, no smoke, does not burn oil and goes fairly well for what it is.

Now I'll attempt to describe the problem;

On a cold start after about 30 seconds of running it develops a rattle/tap. It never does it straight away. It will last for about another 30 seconds then it goes away. Sometimes it will come and go for another minute if I give it a little rev.
The noise does not slowly come in and then slowly fade out, it is either there or it gone completely. Even when it is happening it it is not consistent, it will be rhythmic for 10 seconds, completely go for a few seconds then it will be back again then it will disappear all together.
Once its gone everything sounds and performs completely normal.
It never does it on hot start.
It is coming from the rear of the engine and possibly slightly to the R/H side.

I first thought hydraulic lifter but there are a couple of things that steer me away from that.
-In my experience hydraulic lifters normally rattle as soon as the engine starts and generally fade out as they pump up. They are also normally rhythmic, not rattle hard, completely stop, rattle hard, then go.
-It also seems to me to be a bit faster than lifter speed.

I have had the rocker covers off and run it- no sign of a loose rocker although the noise came and went so quick I may have missed it.
I am still thinking probably a lifter simply because I cant think of anything else.

I thought I would throw it out there for comment before I go deeper into the engine.

Thanks

Dan

[bee1971]

Factory Heads

Simple job of pulling the valve covers

Remove the rocker arms and shafts

Pull out the push rods

Either a extension magnet for lifters or a lifter extractor tool

Shouldn't have to pull the intake unless you have a stubborn lifter - Varnish or wiped out lifter and cam lobe

Start there first and at least you can check the face of the lifters and cam lobes and

Look over lifters

[redgum78]

Thanks Bee71,
I think I might have found the problem. I'll explain and see what people think.

As it only happens on start up and cold it is hard to replicate, I get about 1-2 minutes each cold start then that is it for the day. Anyway each time I cold start it I have had the bonnet (hood) up, air cleaner off and go looking for the the noise as it comes in. The noise is effected by engine speed but it is NOT proportional to engine speed like a lifter or other mechanical rattle. I am now thinking it is something related to the vacuum system. Like a valve chattering but not as fast, a slow chatter if that makes sense. I think once the auto choke plate opens the vacuum drops a bit and it goes away? I started looking at things around the noise area that are vacuum controlled and found the EGR valve. I disconnected and plugged the vacuum hose to the EGR valve. So far 4 cold starts and no noise. Normally i would have had it at least twice in 4 starts. Fingers crossed this is the issue or at least something around this system.
Engine is strong and without any other obvious fault.

Anyone else had anything like this happen?

Thanks for the feedback.

Dan

[flyinlow]

Originally the EGR opened at part throttle with the engine warmed up. If I remember correctly.  The engine will run fine with it left closed.

[cdr]

Does it have a heat riser valve on the pass side exhaust manifold, they can make a rattle clicking noise.

[armor64]

may be of no help, but i had a similar issue on my 78 gmc truck, the PCV valve rattled alot until it warmed up (was worse in winter, when i finally found the noise). a new one fixed it completely. pulled it, stuck a finger over the hole and it stopped, removed it and you could hear and feel it vibrate. once warm it was silent, so it was annoying to find too.

[redgum78]

Well its been a couple more weeks and a few hundred miles. I am happy to say there is not sign of the rattle since I disconnected and blocked the EGR valve and vacuum hose  . yep, it does have the heat rise valve on the R/H side and it does rattle a bit but I wired it up while I was diagnosing so it wasn't distracting me. Fairly fresh PCV in it earlier this year but I did check that out before I got to the EGR.

I am really glad I didn't jump to conclusions too early with this, it sounded and acted so much like a lifter initially. It was that one little clue that it was not quite proportional to engine speed that had me questioning if it might be something else.

[Nacho-RT74]

Does it have a heat riser valve on the pass side exhaust manifold, they can make a rattle clicking noise.

Well its been a couple more weeks and a few hundred miles. I am happy to say there is not sign of the rattle since I disconnected and blocked the EGR valve and vacuum hose  . yep, it does have the heat rise valve on the R/H side and it does rattle a bit but I wired it up while I was diagnosing so it wasn't distracting me.

I was pretty sure that wasn't equipped since 73 or so.

[redgum78]

Does it have a heat riser valve on the pass side exhaust manifold, they can make a rattle clicking noise.

Well its been a couple more weeks and a few hundred miles. I am happy to say there is not sign of the rattle since I disconnected and blocked the EGR valve and vacuum hose  . yep, it does have the heat rise valve on the R/H side and it does rattle a bit but I wired it up while I was diagnosing so it wasn't distracting me.

I was pretty sure that wasn't equipped since 73 or so.

Hey Nacho how things mate? Mine certainly has it. It is a 73 mod but build date is late 72.....maybe thats way?