"Clunk" right after starting engine

[mrbigboy]

Been having an issue for the last couple years that has stumped me.
My LC doesn't get driven much, usually only on the weekends and often for short errands around town. Awhile back I noticed a light "clunk/thump" sound right after starting it. The clunk sound happens once, sometimes twice (so two clunks), about a second after starting. This only happens when the LC has sat for at least two days.

Parts replaced in the last couple years have included: reman Denso alternator, Denso fan clutch, radiator, and more recently a reman Denso starter. I originally thought it was the starter solenoid not disengaging, but after replacement it's still occurring.
I do see my battery voltage drop after sitting for awhile, which is to be expected, so I throw a battery charger on from time to time.
Timing belt/water pump replaced at 122k miles, about 10 years ago.

Rig has ~196k miles. Engine runs strong.

Any ideas?

 

[Mike NXP]

I was also going to guess starter related since not much else is moving .

It might pay to remove your serpentine belt and inspect components . Clunking isn't a typical symptom of accessory pulley failing but its worth a check.

After checking , keep the belt off and let it sit a while like usual

Then, go start the truck with the belt removed and see if the noise is still present
This will help you confirm or rule out certain components .your battery light will probably turn on with the belt removed but that's ok


How loud is the clunk,would the average person even notice it ?

Other than a noise , any other performance issues to note ?

 

[RapidRob]

My first thought was maybe a stray cat sleeping under the hood. I have had that happen and it sounded just as you describe.
If its only at startup after sitting I really can't think of anything mechanical that wouldn't happen at least occasionally at another time.
Maybe try posting a video so the the hive mind can try to help. Good luck.

 

[Patron100LC]

A crusty Ac compressor kicking on after start? Turn AC off before shutting down just to rule out.

 

[Mike NXP]

A crusty Ac compressor kicking on after start? Turn AC off before shutting down just to rule out.

This is a good guess , my highlander makes some stranger noises when I start with ac on, sometimes

 

[mrbigboy]

Thanks for the responses, I appreciate it.

I was also going to guess starter related since not much else is moving .

It might pay to remove your serpentine belt and inspect components . Clunking isn't a typical symptom of accessory pulley failing but its worth a check.

After checking , keep the belt off and let it sit a while like usual

Then, go start the truck with the belt removed and see if the noise is still present
This will help you confirm or rule out certain components .your battery light will probably turn on with the belt removed but that's ok


How loud is the clunk,would the average person even notice it ?

Other than a noise , any other performance issues to note ?

This was something I was thinking of doing as well. The sound is loud enough that the average person would hear it and think WTF was that? No other performance issues.
I pulled the belt last night and everything seemed good to go upon inspection. I started the truck this morning, and no clunk was heard. It had sat from Tuesday until today, which is usually plenty of time between being driven for me to hear the clunk.

My first thought was maybe a stray cat sleeping under the hood. I have had that happen and it sounded just as you describe.
If its only at startup after sitting I really can't think of anything mechanical that wouldn't happen at least occasionally at another time.
Maybe try posting a video so the the hive mind can try to help. Good luck.

This has happened to me with a cat and a rat, and is exactly what it sounds like! No animals however right now, but good call.

A crusty Ac compressor kicking on after start? Turn AC off before shutting down just to rule out.

This may be it.... Seems to make a lot of sense. After reinstalling the belt I started the truck with the AC on. No clunk. I will say that my gut tells me it's something run off the belt, like the compressor, or the alternator, so this is a good assumption.
I'll drive it a bit today, and then let it sit all week. I'll leave the AC off and see if the noise returns after it's sat. I will take a video and see if I can catch it next time. Will post my findings.
Thanks again.

 

[Mike NXP]

Given the thought of the AC clutch possibly being the culprit .... You could also consider just hanging around the engine bay while someone cycles it on and off , see if the noise is heard.

I figure if its coming from the ac it would happen everytime it was engaged ?

If not, then dive deeper .

Did the tensioner feel ok when you removed belt ? Plenty of tension ,no binding ?

 

[pwil393]

Can it be a spark plug(s)? I believe I read a similar description on one of @2001LC thorough threads. Just throwing it out there. Hope you find the issue.

 

[Mike NXP]

Can it be a spark plug(s)? I believe I read a similar description on one of @2001LC thorough threads. Just throwing it out there. Hope you find the issue.

Not a loud clunk described as "making others say wtf"

Would be more of a tapping noise if plug related .....generally speaking


The belt being removed and the noise going away is a good start for troubleshooting....Unless the clunk isn't consistent enough to use that test

 

[pwil393]

Not a loud clunk described as "making others say wtf"

Would be more of a tapping noise if plug related .....generally speaking


The belt being removed and the noise going away is a good start for troubleshooting....Unless the clunk isn't consistent enough to use that test

Agreed, I went back and looked at spark plug thread and the sound is described as a tick and pop. GL OP!

 

[mrbigboy]

Confirmed this morning that the clunk sound is coming from the AC compressor. Started this morning for the first time since Saturday. Had the AC off, and it sounded fine. As soon as I turned the switch on, there was the clunk!

@Patron100LC thanks for the comment, I owe you a beer!

 

[Patron100LC]

Beer is always welcomed. Glad you figured it out.