Knocking sound from under car when going slow, what are the likely causes?

Question

I am getting a knocking car from under my car. It seems to be under the clutch and on the left of it(note we use right hand drive).

The knocking sound is noticeable when you drive slowly e.g. moving off and under 30 mph but you can't hear anything when going fast.

Mechanic 1 says anti roll bar or ball joints. Mechanic 2 says nothing wrong with roll bar or ball joints but rather its the driveshaft by exclusion since the sound only comes when driving. A friend of mine said he had exactly the same thing i.e. the sound is only under 30mph but then goes, they diagnosed him as a wheel bearing and fixed it.

What might be potental causes for my issue, since its the same as my friend had, are these the symptoms of a faulty wheel bearing?

Answer 1

Your issue is probably connected with driveline. I will assume that it is rwd/4wd car as you are suspecting driveshaft problems.

To diagnose whether it is driveshaft related problem, drive slowly next to a wall (road separators are good) in neutral and "measure" the frequency of the knocks by hearing.

Differentials usually have a ratio from 1:2 to 1:4 so if the frequency of the knocking sound is at least 2x bigger than frequency of turned wheel (i.e. occurs at least 2 times per one wheel turn) it is driveline problem: driveshaft joint or its support (I had a case of fuel tank heat shield scratching the driveshaft). If it occurs about once per wheel turn, the problem lies somewhere at the end of driveline: wheel bearing or bent brake disc. A severely worn wheel bearing can be felt by lifting the cars wheel and jiggling it vertically a bit - it shouldn't have a play.