2012 g37x, awd, ~100k

When going over bumps at low speed (<30) i hear a light knocking coming from somewhere in the rear. Definitely a knock sound, not a clunk, and steering/turning has no impact on it.

Bushings look in good shape and I can't get any suspension piece to move abnormally by hand other than the sway bar slightly.

I replaced the sway bar end links, put in new sway bar bushings and also replaced the shocks with new KYBs (old ones weren't leaking but a bit slow on the rebound) and new top hats to rule out the most common items, but no improvement.

Before i bring it to a shop to get another set of eyes on it, curious if there is anything else I should try/look at it, because I'm stumped.

 

What's in your trunk? Is the spare tight to the floorboard?

While under the car, did you lift the exhaust to see if it hits the underside? Rubber hangers get old.

 

can't get the exhaust to bump on anything by wiggling it by hand; checked inside trunk and spare tire was tight

i did have both rear quarter panels cut out and replaced with new OEM ones due to rust, but this knocking started a year after that so I don't think I can attribute anything in the repair to that (unless there's something that gets bolted to the rear quarters behind all the panels that's loosened)

 

When you say "bushing are in good shape" does this include the diff bushing? Its a common failure point, and could explain the noise.

 

I looked at the diff bushings and they visually seem okay. I tried pushing the differential around with my hands and a pry bar and there's no visible play I could see. My understanding is the diff bushings are liquid filled so i'd imagine there'd be visual signs as well; also, I have no issues (vibration, etc.) during acceleration or braking, just going over bumps

I used a pry bar on all 4 corners and articled the suspension pieces every way I could think of and couldn't get any unusual noises or slop

The weird thing is I rode inside the trunk (closed) and couldn't hear any noise, but the driver could hear the knocking. I also ran beside the car and couldn't hear anything while the driver could hear knocking

 

Nope, sounds like you checked it about every way possible, and you're correct about seeing the fluid from a blown bushing. There would almost certainly be residue on the rear subframe.

 

I spent probably 8 hours going over every square inch of the underside on my 4 post lift, 2 post lift and rhino ramps to simulate every possible condition and putting my pry bar on everything and i'm stumped - visually everything looks fine, and everything feels fine (no slop)

The only thing I'm not entirely certain of is the half-shafts. In the rear, if the suspension is unloaded I can pull them slightly left-to-right. When the suspension is loaded I can't move them left-to-right, they're solid. In the front, when the suspension is loaded, I can move them left-to-right slightly. Is that normal?

 

Approximately how much linear movement/ axial end play are you witnessing? Any lateral play? The inboard CV joints are designed to allow some end play to account for suspension travel, however, the outboard joints should be solid. Unfortunately, the FSM doesn't specify axial end play limits for the axle shafts, however, I would say that any movement more than a few millimeters could be considered excessive especially for the rear halfshafts.

IMO, annoying as the sound may be, I'd be inclined to leave it be.

 

Not sure how to measure the play, but it isn't a lot, and the play feels even left vs right axles, and front vs back axles .... to me it seems normal, but I'm not a mechanic so I don't know, but it's the only observation I found of a part with moment.

I think I am going to end up leaving it - with the radio on you don't hear it - but I booked an appointment at a shop to get their view because maybe somebody more experienced than me can pick up on something.

 

So many ideas. Here's another one... Motor mounts? I'm thinking weak mounts would be more noticeable at slow speeds, like you said up top.

 

There is a perceptible difference in movement between a few millimeters and a say a eighth/ quarter inch or more. Being all four (4) axles appear the same likely means they're fine. Only other thing I can think of would be the diff stubs or the wheel bearings.

Again, this is a rabbit hole that is not worth the hassle. Now if the sound gets worse or other symptoms arise then you can delve deeper into it. Just my 10¢ mind you. Good luck.

 

replaced front wheel bearings early last year (Moog) and when wiggling the tire at 3/9, 6/12, etc. it's rock solid, so unlikely

I've temporarily thrown in the towel and have an appointment at a shop to get a third set of eyes on it because it's something i'm clearly missing

did not explicitly check but the engine idles smoothly and there's no unusual vibrations in the car .... and i'm 99% sure the sound is coming from behind me (although i appreciate how sound can travel)

 

...and the rear wheel bearings?

Please keep us updated.

 

rears are still original but the rear tires are solid - i.e., no play when grabbing the tire and shaking it at the various positions - so it seems unlikely

 

Problem solved - it was the front sway bar end links.

I swear to god the noise came from the rear and never once did I think it was from the front. I had checked them and the front sway bar and end links were tight; I couldn't get any play by hand or with my pry bar. However, the mechanic I brought it to showed me that at one angle, when prying on them with the pry bar, there was a very slight knock. And only from one specific angle.

I was skeptical but changed them out and problem completely solved.