Knocking Noise

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Ian

Guest
I have a 99 DII that has a intermittent knocking noise that can only be heard from inside the cab. The sound appears to be coming from the vicinity of the center console where it meets the dash. At first I thought it was the heater fan as the sound is very similar to a heater fan motor, but the sound occurs when the climate control is off. It cannot be heard from the engine compartment nor can it be heard from underneath the truck. I put it on a hoist today to see if I could hear it with a stethiscope on the bell housing and the oil pan...no luck. For no apparent reason it stops and starts. It frequently can be heard after coming to a stop at a light or in the driveway after driving. The sound will get louder and quieter and also will change frequency (speeds up and slows down) while at idle so it doesn't seem to be linked to the engine idle. On a couple of occasions it has sped up when going into gear.

This is driving me nuts. Any ideas?

Thanks
Ian
 
Andrew Homan

Andrew Homan

Well-known member
Jun 7, 2004
3,682
0
Alaska
Anything loose in the exhaust, the internals of the cats or muffler? Is it only at idle or all the time. Does it change durring acceleration?

Cheers Andy 95 d1
 
I

Ian

Guest
I tried the racks this morning. They seem to be solid. The sound does not seem to be directly linked to RPM's. When I was under the truck I could not hear it, so I am assuming it isn't coming from the exhaust.

My big fear with this noise is if it is coming from my ****** or transfer case. Given where the sound is coming from inside the cab that would put the noise in the area of the torque converter.
 
I

Ian

Guest
One other thing I forgot to mention. The sound is faint. If the doors are open or the windows are open it is near impossible to hear. When I don't want to worry about it I turn the radio up.
 
I

Ian

Guest
For anyone interested, the mystery knock is solved. It became progressively worse over the weekend, so I took it into a shop this morning. After about 20 minutes of using old fashioned diagnostics (ear and fingers) the problem was solved.

There is a purge valve for the evaporative emission system on the left hand side of the motor. It has a very faint ticking noise when it runs. The valve shuts on and off and changes frequency. The mechanic actually felt the pulse before hearing anything because in the engine compartment you can barely hear the tick. Anyway the line coming from the purge valve that was routed down between the engine and the firewall was resting on an air conditioning line. The air conditioning line was transmitting the tick to the airconditioning evaporator in the cab, which magnified like a drum changing the tick to a louder knock. One tie strap at no charge and the problem was fixed.

Next time I'm going to just turn up the stereo :D
 
K

ki11erpartybern

Guest
Did it have anything to do with your heater by chance? I think I have the same knocking noise in my Disco but my heater does not work. Air is blowing ice cold but no heat.
Thanks!
 
N

nytefog

Guest
So happy to hear the news you found the "tick". I been trying to figure put where this tick comming from in my 99 D2 as well. I'll have to check things out and see if its the same problem that you discovered. THANK YOU!!
 
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