my truck makes this thunk like clockwork, happens everytime within five seconds of the ignition being turned on whether the motor is started or not. however, i'm not particularly concerned about it, was just wondering what it was. but i'm with the guy who attributed it to the ABS...

 

When I first noticed the "thunk," I assumed it was some sort of ABS thing. Since I know nothing about ABS, I didn't know whether this noise was good or bad. I don't remember how I decided it was the ABS, probably because I watched the idiot light go out when I felt/heard it. The reason I never noticed my '00 making this "thunk" was probably because I was so concerned with the rough idle. This '01 purrs like a cat.

 

opie,

i've had cars that made a thunk/crunch sound when the abs adjusts and self tests at startup...so i think you are probably on to something. maybe i should look in the owner's manual...now there is an idea!

 

That Darned Cat!

 

My truck has done the same thing,hasn't done it in a while though,i think because i drive it more frequently now,it sounded to me like it might have been sucking air up into the intake,but i guess not if yall have had it happen without starting the engine.i never took it to the dealership to get it fixed,they pissed me off with the radiator thing(the normal size radiator with the towing package) and from then on i've tried to just figure it out myself

 

Mine thunks too. The only time I've noticed it is when the AC is already turned on when the truck is started. Seems (probably my imagination) worse the hotter it is. If the AC is turned on right after starting I can feel the compressor start but no thunk. One of these days when it gets worse or something else is wrong maybe the dealer can find it.

 

Are you guys saying without moving it goes clunk???

One more ford is right, the Super duty's blend door clunks badly...sounds almost like a front end thunk...it happens about five seconds after starting.....

Is this what you guys are hearing???

 

Before I went out last night, I turned off my truck's ('99 4x4) radio and a/c. Turned the key to On and listened. I barely heard the fuel pump for the briefest time. Then heard a very slight thunk from under the truck. Is this the thunk you all are hearing 'cause if you are, there's no way I could ever hear that over the sound of a running engine.

 

when i hear it it's maybe half to one second or a little more after the engine first cranks,and you can definitely hear it,even with the radio on

 

as i've posted before, you don't have to start the engine in my truck to get the noise/vibration. it happens within a couple seconds of turning the ignition to the "ON" position whether the engine is started or not. it is actually more of a quick thump/vibration vs a noise. i can sometimes slightly feel it through the steering wheel. i would describe the sensation as almost exactly like the a/c compressor kicking in. but it isn't the a/c compressor since i'll get that sensation even if the climate control is set to "OFF".

 

Hey Troy,

Are you sure the A/C compressor is not engageing? I think my '00 Scab does the same thing you described and I always felt like it was the compressor kicking in just as the engine comes up to high idle. To make sure, have someone else crank it for you and watch under the hood to see if the compressor kicks in. I got a feeling it will even with the climate control off, but hey, I could be wrong.

DaveMan

 

In my last post I said my truck only did it w/ the AC in the on position before starting the truck. Yesterday it made a liar of me. Started the truck, turned on the AC and immediately the thunk happened. Seems to occur at the AC clutch engagement on mine.

Even as deaf as I am I can hear and at least imagine I can feel it over the radio and the cat-back

Mach1 - mine's a 150 not a SD. May still have a connection with the blend door though for I don't remember if I'd changed the temp setting. Maybe I had and the door was moving to full cold.

 

daveman,

mine thunks even if the engine hasn't been started. i seriously doubt the a/c compressor would kick in if the engine isn't running, right?

 

Troy, didn't mean to imply anything stupid, just know that some cars and trucks use the A/C compressor year round no matter the outside temp becuase it helps reduce the humidity in the cab and helps in the winter too beleive it or not.

I'll check it out tomorrow and let you know what I find.

BTW, I think you need to look into that clunk when the engine is off, that sounds kinda weird to me, probably possessed or something.

DaveMan

 

yeah, i've had several vehicles that allow the a/c to cycle as low as 40 degrees, but no lower than that as compressor damage can result. however, i'm positive it isn't the a/c in my truck. i'll have to open the hood and look and listen real intently sometime soon. i truly suspect it is entirely normal, probably the abs doing a self test or something. somebody earlier mentioned something about a transfer case self test but that sounds like a load of crap to me.