Please help! …. What is this tap/knock noise?!

CamGTP · December 25, 2021

For sure not a misfire, the engine is running smooth as silk.

Something is being whacked around or at least sounds that way. I was going to suggest removing the serp belt to see if something on that would be the cause but it seems to be coming from the bottom of the engine or between the engine and transmission. I mean you could still remove the belt and start it up, it can run off the battery and have no water pump flow for a tiny bit. It would at least let you know if the noise was still there.

My other thoughts are to remove the little inspection cover on the bell housing of the transmission to make sure nothing is bouncing around inside of there. Like a flex plate bolt. Also just being under the truck will really let you help track down where the noise is coming from.

Supreme Pizza · December 25, 2021

5 minutes ago, CamGTP said:

For sure not a misfire, the engine is running smooth as silk.

Something is being whacked around or at least sounds that way. I was going to suggest removing the serp belt to see if something on that would be the cause but it seems to be coming from the bottom of the engine or between the engine and transmission. I mean you could still remove the belt and start it up, it can run off the battery and have no water pump flow for a tiny bit. It would at least let you know if the noise was still there.

My other thoughts are to remove the little inspection cover on the bell housing of the transmission to make sure nothing is bouncing around inside of there. Like a flex plate bolt. Also just being under the truck will really let you help track down where the noise is coming from.

If there are no OBD2 does, then we can rule out a misfire.

Bell housing inspection is an idea. Could be a Christmas tree ornament hanging from the radiator, being whipped up by the cooling fan.

Bender9406 · December 25, 2021

12 hours ago, CamGTP said:

For sure not a misfire, the engine is running smooth as silk.

Something is being whacked around or at least sounds that way. I was going to suggest removing the serp belt to see if something on that would be the cause but it seems to be coming from the bottom of the engine or between the engine and transmission. I mean you could still remove the belt and start it up, it can run off the battery and have no water pump flow for a tiny bit. It would at least let you know if the noise was still there.

My other thoughts are to remove the little inspection cover on the bell housing of the transmission to make sure nothing is bouncing around inside of there. Like a flex plate bolt. Also just being under the truck will really let you help track down where the noise is coming from.

You might be onto something- when I was under the other day I saw the inspection plug missing from the housing. And also the side plastic coverings on the sides of transmission were loose.

So maybe transmission work was done and there is a bolt in there tapping or knocking around…. Idk

Next time I hear it I’m going under and seeing what I can see with it running still.

Vaccum pump and something with belt or pulley could also still be an issue. But only once it warms up. I do have a rather stiff break peddle too now that you mention it.

shakenfake · December 26, 2021

On 12/25/2021 at 12:02 AM, Supreme Pizza said:

If there are no OBD2 does, then we can rule out a misfire.

Disagree had a misfire in two cylinders on my truck but no codes haha but yeah I wouldn't say misfire

Supreme Pizza · December 26, 2021

34 minutes ago, shakenfake said:

Disagree had a misfire in two cylinders on my truck but no codes haha but yeah I wouldn't say misfire

No codes on a two cylinder misfire? What year of vehicle?

They must have been random misfires on those cylinders, not steady misfires.

How did you track them down without codes?

shakenfake · December 28, 2021

99 5.3 Silverado. Definitely steady misfires, had two bad injectors

Supreme Pizza · December 28, 2021

Huh. No check engine light, but were there any pending codes in the computer?

shakenfake · December 28, 2021

Yeah for a bank 2 O2 sensor. I can't remember now but I actually got the code to go away by swapping in a new MAF. Obviously didn't fix the misfire as I could feel it.

Bender9406 · December 29, 2021

Truck is back at dealership so I will let you know what the verdict is… it actually hasn’t made this sound in a week since I took the video.

Bender9406 · January 13, 2022

So. The answer was …. Thermostat! Faulty thermostat. That knocking and tapping only started when thermostat got to temp - and continued to make noise after engine was off.

$50 from Auto Zone and you get the thermostat and housing unit in 1.

Bender9406 · January 13, 2022

https://drive.google.com/file/d/1KoqKtUDVdsJEYxBm7jxRX3q82TtU4r3y/view?usp=drivesdk

Bender9406 · January 14, 2022

Or so I thought ...

Noise came back! That hellacious hammering, knocking, noise. It did go away with new thermostat, but after a day it returned. The good thing is, the knocking after the engine is shut off is gone. So maybe it did need a new thermostat, but that didnt rid my problem entirely.

You can feel the surging going on and out of the block in the lines that go to the heater core. You can also see the surging on the bottom and top radiator hoses. Water pump is new.

Is there a special bleed procedure? Will a bad surge tank cause the surge/knock Im experiencing? Maybe air getting into the lines? Oil looks OK so I dont suspect head gasket.