2001 Ford Mustang GT knock noise

My buddys got a 2001 Ford Mustang GT with 3.81 rebuilt motor. When he accelerate's a slight knocking noise comes from the left hand side of his motor. He said it just started about 3 days ago. Im thinking the timing chain might be going bad. But I dont know anything about Mustangs. So I figured I would ask you guys. If anyone could get back with a possiable answer as soon as they can. I would be thankful.

If its a GT it will have the 4.6l v8. The 3.8l is a v6. How loud is the knock? Is it speed specific? Does it only happen during a certian window of rpms or is it always there and you can just hear it better when he accelerates? Is it a hollow sounding knock or a more solid knock? Does it make more of a rattle or a definate knock? Need more info.

If its a GT it will have the 4.6l v8. The 3.8l is a v6. How loud is the knock? Is it speed specific? Does it only happen during a certian window of rpms or is it always there and you can just hear it better when he accelerates? Is it a hollow sounding knock or a more solid knock? Does it make more of a rattle or a definate knock? Need more info.
The only thing he told me was its coming from the left side of his car, it only makes a slight hollow knocking noise when he picks up quickly. But he said it dosnt affect his pick up. It only makes the slight hollow knocking noise when he slams on the gas. So Im taking it it only makes the noise at high rpms. But he really didnt tell me much about it. Also with the 3.81 I must have be mistaken with the motor. I no he did rebuild his old motor after he blew the stock one.

Ok, now that we know its been rebuilt this changes a few things. Did he rebuild it himself or was it professionally done? At first I was thinking he may just be hearing spark knock/light detonation. But if its a hollow knock for sure it could be a more serious problem. I doubt however that its "piston slap" because he should hear that all the time. It comes from a broken piston skirt. It could be a wrist pin but he should also hear that more than just when he gets on it hard. If he can verify that it is only a problem when he is running it hard then I doubt its anything in the shortblock. He may just have a loose follower. How long ago was it rebuilt? How many miles since the rebuild?

Ok, now that we know its been rebuilt this changes a few things. Did he rebuild it himself or was it professionally done? At first I was thinking he may just be hearing spark knock/light detonation. But if its a hollow knock for sure it could be a more serious problem. I doubt however that its "piston slap" because he should hear that all the time. It comes from a broken piston skirt. It could be a wrist pin but he should also hear that more than just when he gets on it hard. If he can verify that it is only a problem when he is running it hard then I doubt its anything in the shortblock. He may just have a loose follower. How long ago was it rebuilt? How many miles since the rebuild?

Im not sure how long its been rebuilt. I think its been almost half a yr now. And he had it pro rebuild, he said he had alum headers and blocks put on bigger cam. I personaly have not heard the knock, Im just telling you what he told me. The knock has only been going on for the last 3 days he says. He also says he can only hear it when his window is down. He cant hear it with his window up. And as far as I no he said its a hollow knock. What if its a soild knock? What could be the problem then?

A solid knock could be a rod.

Is he sure its from the motor? Will it make the noise in neutral? If so then that could help pinpoint the exact area the noise is coming from.

A solid knock could be a rod.

Is he sure its from the motor? Will it make the noise in neutral? If so then that could help pinpoint the exact area the noise is coming from.

He said it only makes the noise when he accelerate's, it dosnt make the noise in netural. And he thinks its coming from his motor, but he cant tell because it only makes the noise when he drives.

Tell him to go over the front suspension very carefully. It could be as simple as a ball-joint or something. Does this knock happen as long as he holds the gas or is it just one hard knock as soon as he floors it?

Tell him to go over the front suspension very carefully. It could be as simple as a ball-joint or something. Does this knock happen as long as he holds the gas or is it just one hard knock as soon as he floors it?

He said its a constant sound when he holds the gas to the floor. I'll have him look over his suspension.

Have him look at the spark plugs to see if there is a misfire.
What grade of oil is he using?

Have him look at the spark plugs to see if there is a misfire.
What grade of oil is he using?

Not sure what kind of oil he's using.

I'm thinking he may just be hearing spark knock.

Could be a lot of things. Maybe a long shot but check the tensioner pulley and make sure its not starting to fail. Mine is making a similar noise and thats what it is. The pulley is loosing some tension and starting to move at times.

He got it fixed, It was a gasket, Cant remember which one.

Only gasket I know of that makes a sounds someone could consider to be a knock would be an exhaust manifold gasket. But who doesnt know what an exhaust leak sounds like...