Knocking Vibration noise

Hey all, long time lurker - first time poster.

I've owned my Aveo T300 1.3 VCDI (2012 Model) for about 6 months now, bought it used at 39,000 miles and just done about 3,000 since. About 2 weeks after I bought it, it started with this plasticy vibrating, almost knocking noise coming from the front drivers side (UK, so right hand side) whenever I go over roughish surfaces or if the engine is struggling at low revs in 3rd and above. If I go over any sharp bumps, like potholes or cuts in the road it rattles loudly. When in the drivers seat, it sounds like it's coming from all around you but when in any other seat it's clearly coming from the front-right. The weird thing is it doesn't happen just going over smooth bumps or speed bumps so I don't think it's my running gear. When on smooth tarmac/asphalt or at speed on the motorway the noise isn't present. The noise is usually there but the volume varies day-by-day.

Had it in to a mechanic who said everything seemed sturdy and tight but a typical issue was the plastic Anti-roll bar links so had them replaced, issue still there.
Had it back in and he looked over my suspension and wheels - said there was slight play in my shock absorber caps but advised me against doing them as they weren't bad and he couldn't see how that would cause the noise I was describing.
Had it back in again and finally got him to actually drive the car so he could hear the noise for himself and he Suggested that my lower ball joints were badly corroded and got the dealership to replace them, issue still there.

I'm tearing my hair out at this point - I've looked through the forums and can find little bits and such about rattles and knocking noises but they don't quite match the symptoms in my car or suggested fixes have already been made. Really want to avoid taking it back to the mechanic as he's written it off as something he can't figure out and the dealership I bought it from refuses to acknowledge there's even a noise.

As a side note, my front rubber diffuser lip on the underside of the front bumper is off slightly where it took a corner at the bottom of a particularly steep hill and broke a couple of clips- I thought this could be causing it but the clips are broken on the left side, not the right where the noise is coming from. I tested it out by duck-taping the diffuser just in case and taking it for a short drive but that made no difference, so I'm completely at a loss now!


If anyone has any experience with this or any suggestions it would be much appreciated!

UPDATE 12/06/17: Ok, so immediately after this post I decided to go out and try and see what the variables were in different conditions to try and figure out where this noise is coming from.

First I noticed the vibration was more prevalent when the car was warm - either after giving it a good run or if the outside ambient temperature is above 18 Degrees Celsius.
Second I noticed the vibration is quieter if it's raining - this could be because I can't hear it as well over the water but thought I'd give it a mention.
Finally and I think most importantly, the noise is abated completely if any pressure what-so-ever is applied to the brakes. I've tested it multiple times and can confirm there is absolutely no knocking noise or vibration at any speed if the brake is being applied.

Anyone have any thoughts at all? it's kinda lonely here :(

I would take apart your brakes and see if anything seems to be missing then. Nice detective work. :)

Ok, so it's been a couple of weeks but thought I'd chime in with an update:
I finally managed to get around to getting the brakes checked by a garage chain who were offering a free brake check (I'm ok with electrics and changing wheels and easy parts etc. but not messing with brakes).

Just had a call from the garage who told me the brake pads last fitted were not the correct thickness and were around 1mm thinner than original spec, which is allowing play on the pad to vibrate and knock against the inside of the caliper when going over rough terrain.

Further to this, they also told me that the front brake rotor/discs were very close to minimum width - apparently 20.4mm to a minimum of 20mm and - of course - recommended that I have both the front discs and brake pads replaced for £220 which I was fully expecting from an autocentre chain offering free brake inspections.

I obviously have the feeling that they are telling me this to get my money but something just doesn't seem to add up here - the car is only 4 years old and 42k miles and even in my old and reputably unreliable MG ZT I managed almost 70K and 12 years on the original brake discs before having them changed.

Can anyone tell me what is the actual recommended minimum width of the rotors for the T300 as I can only find info regarding the mkI an II's online and is it typical to need to need to replace them at the 40k mark?

I'd rather try and make sure that this would be causing the noise I'm hearing and that they do actually need replacing before having to drop any money on them.

Disc brake pads (and usually the rotor as well) are an easy DIY job. If you are comfortable pulling wheels, you would likely have no trouble doing the disc brakes as well. Take a look at a few YouTube videos to see how simple it really is.

Edit: The shop saying the pads have "play" on them because they are thinner doesn't make sense to me. All the brake pads I've ever replaced are clipped into the caliper and being 1mm smaller wouldn't have made a darn bit of difference.