I recently purchased a 2016 QX50 RWD with ~30k miles. I'm experiencing a drivetrain hum/groan when off throttle over about 40mph. 99% sure it's not tires or wheel bearings. It's relatively quiet, but can be heard in the background of music, rain, etc. This happens in gear or in neutral, and under about 40, it's as quiet off throttle as would be expected (no noise at all). The dealer replaced the rear diff, but that did not solve the issue. Has anybody here had a similar issue? The car is CPO and has factory drivetrain warranty, which is nice. The dealer is going to look at it again.

This may or may not be related, but at least compared to other QX50 3.7s and my old '07 G35, the idle is a bit rough. The steering wheel vibrates, I can feel it in the pedal, dash, etc. The vibration goes away around 900-1,000 rpm. Idle is at 600-650. Is this normal for the 3.7? I know it's not a particularly smooth engine, but this seems excessive.

Thanks

The rough idle is certainly a concern - should be almost imperceptible at idle. Anyone saying otherwise has $$$ to lose.

I'm not positive it's associated with the other issue - the fact that it happens in N and D leads me to believe its a brake dragging (I'd think wheel bearing as well, but at 30k miles its unlikely).

Keep us posted on what the dealer tech says - make sure you go on the test drive to help ID the issue.

Thanks for the reply. I figured the idle was an issue. The old VQ35HR in my G35S was smooth at idle, even at 135k. I starting to think that I should have kept it. Heh.

The master tech rode with me before the differential replacement, and he did hear the noise. They ruled out wheel bearings. The noise has not changed, and he is going to ride again to verify. I wonder if it's related to engine or transmission mounts, or maybe the CVs or driveshaft/carrier bearing. They seem a bit stumped.

Does the transmission disengage anything below 40-ish? It almost feels like it freewheels or something like that below that speed, and that is when the noise goes away. Could be unrelated though.

Speaking of brake dragging, or at least sort of, with the windows down, a "squishing" noise that's related to speed seems to be coming from the left front wheel. I would assume that's related to the recently installed tires, but maybe not.

Not sure when automatic trans 'disengagement' occurs in a modern trans - it has to do with line pressure, not speed (to my understanding).

A fluid sample wouldn't be a bad idea. Not sure if that car has a carrier bearing (I don't think it does).

Does the noise fade gradually or does it cease abruptly as the car loses speed?

Not sure what to make of the last comment... "squishing" would be a new one for me. Swishing? Like a broom sound, increasing with speed?

Heh, yeah, meant to type "swishing". Frequency does increase with speed.

The hum/grumble drivetrain noise fades quickly from clearly audible at a touch over 40mph to nothing at a little under. At that point or a mph or two lower, it feels like something disengages in the transmission and the revs drop to around 1,000. It generally seems to get louder at higher speeds, but it's hard to tell over the increased tire/wind noise. This is all with zero throttle. It seems like it goes away with throttle, regardless of speed. Looking at a parts diagram, it looks like there is not a carrier bearing.

Since this has a warranty (and it seems like this could be an expensive repair), I'm not going to touch anything. I'm curious what they decide to try next.

Oh - did these come with acoustic windshields from the factory?

Swishing is easy, my guess is a warped rotor or something in the brake or hub assembly is awry. I'd set that one aside for them to address later.

If the sound disappears w throttle input (load), that's typically indicative of a differential issue (incorrect meshing of the gears). Since they replaced the diff, the next thing to consider is the transmission. That last description was much more... descriptive.

Yes, acoustic glass for side windows and windshield.

It has new pads/rotors, but yeah, I'll have them check it out. I'll add it to the list. Heh.

Transmission is next in line, I agree. We'll see. I drove it some more today, and the noise is mainly between 40 and 50, and seems to fade away over that. I didn't get it up to highway speeds.

The dealer also put in a new windshield, and I'm trying to figure out if it's acoustic. It's a Pilkington. There's no obvious marking that other acoustic windshields have, though it does have a lone "A" under the other info. Apparently this can mean acoustic sometimes, but I have no idea.

To update this one, the culprit was indeed the u-joint on the driveshaft. It took a while to get the new parts: the driveshaft had to be manufactured, was damaged in transit, had to get another one... took 2 months total. But hey, the noise seems to be gone and I had a loaner. They also did the front wheel bearings, just to be sure... and the steering wheel adjustment motor, because that was grinding.

The idle is still a bit boomy, though they said that is normal. Nothing really vibrates anymore, just a low frequency sound. I'm not sure it is normal (at least it shouldn't be), but after waiting a total of 3 months for this process, I decided to not press the issue and deal with it later. We'll see if it smooths out with time. Also, is a very slight whine normal upon initial acceleration in 1st? I don't know if it's from the transmission or engine, and goes away by 5mph or so. It sounds sort of like a Honda automatic whine, but quieter and higher-pitched.

I mean, I like this thing and all, but it doesn't seem quite as nice as my old G35S. Maybe it wasn't treated very well by the original owner.

Sometimes when I shifted into D, I heard tic sounds coming from the engine bay. anyone else has these problems?

Hughes W. wrote: ↑

Sat Apr 18, 2020 8:59 am

Sometimes when I shifted into D, I heard tic sounds coming from the engine bay. anyone else has these problems?

Could be axle splines.