Strange clonking noise immediately after a bump

BMW i3 Forum

Help Support BMW i3 Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Nov 17, 2019
So, my first post is an issue and I think a solution to my problem.

Car: 2014 Rex with all options except for external sounder (bar the Rex inter-changeable options). Bought second hand at 47K miles (now with 58K), had 10 months of BMW approved used warranty remaining (12th December 2019 it expires). Had the stuck charge cable servo issue fixed. A steering column with evasive play replaced. So far I'm being a little pedantic about rattles and issues as its under warranty, did have a strange steering issue last week where is appeared to keep going around a corner and the steering seemed to go a little stiff. Shall be going back to BMW next week to get them to have a look at this, the steering issue and if they can get the rubber on the door handle that seems to of dropped off. So onwards to my technical problem and solution (hence why its under technical discussion as I've already found the issue, fingers crossed) ...
Issue: Car has a strange dulled clonk (I think that's an onomatopoeia) immediately after a bump, which is especially pronounced when going over a bump on the left hand side. Seems to of got worse over the last 1000km or so.

Investigation: Many many hours of looking, prodding, wobbling, pulling and tapping in the front end. Found it much easier to investigate with the frunk box and the protective panels removed from the front.
There were two possible areas that I thought had excessive play/travel, the two expansion tanks imho had poorly designed brackets. One is rigidly mounted (Figure 1) and will have accelerated life on hoses and the bracket itself (will be a place to watch for corrosion). The bracket strengthening gusset is only on one side causing a twisting moment when subject to shock. Its a light weight bracket which I appreciate given this cars credentials, although the choice of two expansion tanks has confused me slightly.


Figure 1

The second (Figure 2) is what I believe to be the source of the sound. The top of the tank is bolted with a torx (Where the '3' terminates) and two positioning lugs either side of the torx to hold it in place. However the bottom is held in place with a rubber grommet. I believe the rubber grommet has worn and is no longer offering any friction on the tank to hold it in place. As you go over the bump it lifts out of the grommet and then slaps against the grommet once over the bump. Would also result in a potentially damaged expansion tank over time. Its also a 'MINI' branded part.


Figure 2

Unfortunately I can't upload a video of either moving, but as an interim fix I 'shoved' some shop towels in there to pad out the movement. After a couple of clicks around the block (2km around the local area) it seems to be much better. However I would like to get a new grommet from BMW but I can't for the life of me find the part number. Will know for sure when I drive to work tomorrow (48 mile commute)

Solution: Shoved in shop towels. Image to follow.
So my fix was just confirmation bias on my part. clonk is still there. I'm wondering if its the ball joint but not really sure how to check.
I had a similar sounding issue on a different car. Turned out that the top of the strut had come loose (stripped out). My strut was held in place by the weight of the car. Putting the car on a lift revealed the problem.
Bmw has replaced my front strut tops but the clonk remains. I shall do some investigation this weekend as I have some more ideas where the source could be.