# Hard metallic knock when starting to drive **URL:** \ **Category:** Maintenance/Repairs\ **Tags:** m-class, mercedes-benz\ **Created:** [February 6, 2012, 9:22am UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818 "2012-02-06T09:22:45Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![Abiredu](https://avatars.discourse-cdn.com/v4/letter/a/67e7ee/32.png) [@Abiredu](https://community.cartalk.com/u/Abiredu)\ **Post date:** [February 6, 2012, 9:22am UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/1 "2012-02-06T09:22:45Z")
I have a 1998 Mercedes ML. In the past few weeks, there is quite a loud metallic knock when I start driving. It does not happen if I take off real slowly. What could be the cause? ---
**Author:** ![VDCdriver](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/vdcdriver/32/30_2.png) [@VDCdriver](https://community.cartalk.com/u/VDCdriver)\ **Post date:** [February 6, 2012, 4:43pm UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/2 "2012-02-06T16:43:11Z")
The first thing that I would check is for a bad u-joint on the drive shaft. What you describe is a classic symptom of a failing u-joint, and on a 14 year old vehicle, it would not be unusual to have a u-joint (or two) worn out. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [February 7, 2012, 9:55pm UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/3 "2012-02-07T21:55:16Z")
Is it a single knock or a repetative knocking sound? I agree with VDC if it’s a single knock, but if it’s repetative that would suggest other possibilities. ---
**Author:** ![ok4450](https://avatars.discourse-cdn.com/v4/letter/o/e9bcb4/32.png) [@ok4450](https://community.cartalk.com/u/ok4450)\ **Post date:** [February 7, 2012, 11:20pm UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/4 "2012-02-07T23:20:12Z")
I’m in agreement about a driveshaft problem and if not a U-joint it could be a center carrier gone bad. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [February 7, 2012, 11:53pm UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/5 "2012-02-07T23:53:05Z")
This specs out as 4WD, 4 wheel independent suspension. So many CV joints (perhaps double cardon joints in the rear??). So many possibilities. ---
**Author:** ![Somua](https://avatars.discourse-cdn.com/v4/letter/s/9de053/32.png) [@Somua](https://community.cartalk.com/u/Somua)\ **Post date:** [February 20, 2012, 6:48am UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/6 "2012-02-20T06:48:22Z")
Thanks guys. I’ll have it checked out. Do I assume its not going to break down on me sometime soon.? ---
**Author:** ![ok4450](https://avatars.discourse-cdn.com/v4/letter/o/e9bcb4/32.png) [@ok4450](https://community.cartalk.com/u/ok4450)\ **Post date:** [February 20, 2012, 11:07am UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/7 "2012-02-20T11:07:51Z")
You should not assume that. A faulty CV joint, U-joint, or center carrier (assuming this problem is one of those) can come apart at any time. I would think that if the problem were drivenline related though that there should be some more symptoms such as a vibration, shudder, buzz, etc. ---
**Author:** ![the\_same\_mountainbike](https://avatars.discourse-cdn.com/v4/letter/t/c89c15/32.png) [@the\_same\_mountainbike](https://community.cartalk.com/u/the_same_mountainbike)\ **Post date:** [February 21, 2012, 10:33pm UTC](https://community.cartalk.com/t/hard-metallic-knock-when-starting-to-drive/70818/8 "2012-02-21T22:33:40Z")
I agree. There are too many possibilities here that could come apart on the road and cause total loss of control. Assume nothing. Your safety may depend on it.