Thunk noise on start - W222 S550
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2015 S550, 2017 GLS450
Thunk noise on start - W222 S550
For the past few days, I've left the car parked outside where the temps are dropping to around 35 to 40 degrees F. Not quite freezing.
Within a second of starting the car, I hear and FEEL a thunk. It's louder than in the video and I can feel the vibration of the THUD in the cabin. It's as if something physically slammed against he engine bay/firewall.
Today at around noon, the temperature outside was 60 degrees F and the engine temperature was 120F upon starting. But even before I started the car, I opened the rear door to load some stuff in the car. Within a few seconds of opening the door (before I actually got into the driver's seat), I again heard the THUNK. Now I'm pretty sure it has to do with something happening during the automated cycling routines.
Anyone have any ideas? Car is driving beautifully and I'm not having any issues except for the aberrant noise on cold starts. And now noticed even before engine ignition.
Within a second of starting the car, I hear and FEEL a thunk. It's louder than in the video and I can feel the vibration of the THUD in the cabin. It's as if something physically slammed against he engine bay/firewall.
Today at around noon, the temperature outside was 60 degrees F and the engine temperature was 120F upon starting. But even before I started the car, I opened the rear door to load some stuff in the car. Within a few seconds of opening the door (before I actually got into the driver's seat), I again heard the THUNK. Now I'm pretty sure it has to do with something happening during the automated cycling routines.
Anyone have any ideas? Car is driving beautifully and I'm not having any issues except for the aberrant noise on cold starts. And now noticed even before engine ignition.
Last edited by bkdc; Dec 18, 2019 at 04:28 PM.
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I just had my ABC pump replaced last week. I did not get any thunk but I did have worn bearing noises that would get louder when the temperature was warmer outside. The suspension worked perfectly but still the pump was bad. I am not suggesting it is your pump but if you start hearing noises like I describe............
One thing you could do if you have the gumption is to release tension on the drive belt and remove it and then start the car cold for a brief time and see if the noises is no longer there. If no noise then you have narrowed your issue down.
Katie
One thing you could do if you have the gumption is to release tension on the drive belt and remove it and then start the car cold for a brief time and see if the noises is no longer there. If no noise then you have narrowed your issue down.
Katie
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2015 S550, 2017 GLS450
Well, I'm just wondering how many things not related to the drive train can cause enough force to make the cabin vibrate with a loud thunk. It has to be some pump or compressor.
ABC pump is a good thought. Also air conditioning compressor was a thought. I heard that clutch engagement could occasionally cause a loud noise
ABC pump is a good thought. Also air conditioning compressor was a thought. I heard that clutch engagement could occasionally cause a loud noise
Last edited by bkdc; Dec 18, 2019 at 07:55 PM.
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If I recall the compressor is NOT clutched. If I recall off the top of my head it is a constant flow variable compressor.
I will also say I have had TWO idler pulleys I have replaced but they were merely wobbling a bit and not making any noise or such.
Like I said it might be a worthwhile thing to take the belt off and start the engine BRIEFLY and I mean briefly to see if the noise is gone.
One last thing comes to mind and that would be a sticky shock that is making that noise when the fluid pressure builds up from the engine running.
Katie
I will also say I have had TWO idler pulleys I have replaced but they were merely wobbling a bit and not making any noise or such.
Like I said it might be a worthwhile thing to take the belt off and start the engine BRIEFLY and I mean briefly to see if the noise is gone.
One last thing comes to mind and that would be a sticky shock that is making that noise when the fluid pressure builds up from the engine running.
Katie
Last edited by Katie22; Dec 18, 2019 at 08:27 PM.
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I am really sorry I know this is not related, but how do you get this small km/hr readings to show up in the right bottom side of the speedometer !???? I don't have this and saw many cars without it but some other cars have it... How can I turn it on?
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it’s In settings accessed through steering wheel.
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2015 S550, 2017 GLS450
The noise magically went away this morning, but after popping the hood, I have a good idea of what was causing it. Once I have time to do a full inspection this weekend, I’ll report back. 
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Full inspection shows no abnormalities other than squirrel droppings on top of the engine and a few on the lower splash shield.. Thankfully no damage and car running well, but this is what I get for leaving the car parked outside for several days. I'm pretty sure the loud thump is a scared squirrel as I heard it several times and the timing varied with my proximity to the car. LOL.
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I just had my ABC pump replaced last week. I did not get any thunk but I did have worn bearing noises that would get louder when the temperature was warmer outside. The suspension worked perfectly but still the pump was bad. I am not suggesting it is your pump but if you start hearing noises like I describe............
One thing you could do if you have the gumption is to release tension on the drive belt and remove it and then start the car cold for a brief time and see if the noises is no longer there. If no noise then you have narrowed your issue down.
Katie
One thing you could do if you have the gumption is to release tension on the drive belt and remove it and then start the car cold for a brief time and see if the noises is no longer there. If no noise then you have narrowed your issue down.
Katie
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