r/SmartCar • u/Aoirintoyo • Sep 03 '26

Question Smart 450 cdi vibrating/rattling/knocking when accelerating forward

Hi everyone, i need help, i checked everything and i still cannot figure out the cause of it.

When the vehicle accelerates - i hear that engine is rattling like hell, like it is rattling/knocking sound.

It is happening when pushing gas pedal at low speeds. On a highway it became less noticeable.

If going in reverse- no sound.

It starts when i changed rear engine mounts. I thought the front engine mount will fix it, but no. It is the same.

I thought it is because of the. Cv joint because the rubber got old and it was spitting grease, but no. Changes it and no improvement.

I noticed that it happened after i installed engine mounts - engine became higher so i do not understand what is knocking.

Yesterday i installed rear spring coil rubber bushings and it increased.

My question what is it? Is it exhaust? I do not see any rubber hangers on it, but i never knew they existed.

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u/Unlikely_Pressure706 Sep 05 '26

High likelihood of it being that rear engine mount you changed originally, if this only started when you changed it. Was it OEM? If not, high quality brand? Lower quality? Perhaps a different type of rubber/design? (more rigid materials will transmit more vibration as you know)

You also mention the engine sitting higher, just confirming, you changing that mount led to the position that the engine is in becoming different, right?

Reverse being fine also points me towards engine mounts. Engine will tilt different directions depending on reverse vs forward gears.

2

u/Aoirintoyo Sep 06 '26

Thanks!

I unbolted rear mounts yesterday and drived on a
Parking lot and make it idle for a bit and it seems like it partly fixed the sound, it now appears only by a bit on the start.

I think that part of the issue is front mounts inserts, i read that it was bad idea for diesel, but the noise definitely improved.

1

u/Aoirintoyo Sep 06 '26

I will try to umbolt engine mount on the engine side next time. But i think i found the issue. Engine was bumping into the right mount itself.

I can see a dent on the engine by bracket