I'm very new to this forum, so I'm not sure if this is even the right place to post this, so excuse me for that.

I have only a modest knowledge of cars so a lot of my knowledge is guess work. I have this issue with my car where when I first start the car and begin driving off slowly, I will here some mechanism and then a whirring metal grinding, then my car will kinda jolt forward a bit, as if shifting a gear, but it doesn't seem to be a gear shifting problem, as my transmission is fine throughout the ride and this is only right after I start driving my car and no other instances. The grinding seems to have gotten more aggressive as some time has passed. The only thing I can think of is maybe some kind of clutch that's engaged at the start and is disengaging, or a mechanism similar to that. But it's an automatic transmission and I'm not sure of what clutch-like mechanism exists in this specific scenario. Thank you for any information you can provide in helping to diagnose this issue.

 

When you first hit 5mph the ABS system does a self check, that "whirring/grinding" is the ABS motor turning on for 1 second during the self test. It's located directly in front of the driver seat, inside the engine bay on the firewall.

If it's causing engagement however then something else is out of spec, you're not touching the brake pedal at all while it's doing this correct?

I would first check brake fluid level, if it's fine then you probably need to schedule a 1 hour diagnostic at Nissan/Infiniti because something is turning one of your ABS solenoids on causing that jolt you're feeling.

My gut feeling is one of the solenoids is not fully closing and letting pressure leak through during the self test which would literally be causing one wheel to have it's brakes turn on momentarily during the self test.

You can test this theory yourself by disabling the ABS/TCS system, directly in front of the brake booster/ABS block is a black box with 3 harnesses attached to the back of the driver tire fender, unplug those harnesses. The black box is the ABS relay and if it's disconnected the system is disabled and will not self-test.

Then you can go for a short drive and see if it still has that same jolt forward, if it does then it's not an ABS/VDC issue.

Be aware you will have error codes on the dash when the ABS relay is unplugged and your anti-lock brakes, vehicle dynamic stability, and traction control will all be disabled.

Ignore what I'm pointing at in this picture, the red circled area is where the ABS relay is located.

 

Ohh and you should disconnect the battery anytime you are unplugging or plugging in electrical harnesses.

 

Wow, thank you for the very detailed reply. After I made the post, I did find some information that seemed to point to the ABS, but I wasn't certain. If it helps, one time I was going downhill after first driving, holding the brake, and when it happened, my brake pedal seemed to jolt a bit under my foot, as if depressing or something. Scared the hell out of me for a split second cus I thought I just lost my brakes. I should have time to check out what you've said this weekend though, while I'm changing my belts.

 

There's quite a few things that can trigger the ABS relay (not including the self-test). The most common culprit is having poor alignment though, it uses a combination of the steering angle sensor, yaw sensor, and wheel speed sensors to determine trajectory.

Steering angle sensor is in the steering wheel, part of the clockspring / spiral cable, it tells the ABS/VDC computer which direction you are steering.

Yaw sensor is a fancy electric compass mounted under the center console, it tells the ABS/VDC computer which direction you are ACTUALLY going.

Wheel speed sensors are mounted on each wheel, they tell the ABS/VDC computer how fast each tire is spinning.

If your alignment is off and you have to counter by turning the steering wheel then it causes a discrepancy between the steering angle and yaw sensor, the computer thinks you are sliding like on snow so it will make corrections to individual brakes to try to straighten you out.

Wheel speed sensors, if the computer detects a difference in speed on one wheel it thinks that wheel is spinning and it will make corrections to keep the wheel from spinning.

Corrective action is basically turning on the ABS motor which builds up hydraulic pressure in the brake system, then it opens up specific solenoids to send pressure to whichever brake caliper needs to be slowed down. This is the "jolt" you are feeling, it's LITERALLY hitting the brakes on one corner even though you aren't touching the brake pedal.

Corrective action can also include pulling throttle. Your gas pedal is just an electrical switch sending a signal to the ECM, not a physical cable, it's a throttle-by-wire system. The VDC/ABS will tell the ECM to pull throttle which limits rpm. This doesn't feel like a jolt though, it just feels like you have the pedal mashed to the floor but the engine won't rev.

If you press on the brake pedal while the ABS motor is turned on it will feel like a strong vibration in the pedal, sort of like you hit a rumble strip on the highway.

 

Another somewhat common issue on these vehicles is a sticking ABS relay, not what you're experiencing but something to be aware of nonetheless. If you turn off the car and you still hear that grinding/whirring sound then the ABS relay is stuck closed. Just unplug it and drive with the ABS/VDC disabled until you can replace the relay. If it runs too long because of a stuck relay then it cooks the ABS motor which is fairly expensive to replace. The relay however is cheap and was used on almost every Nissan from 1995 to 2015 so there are PLENTY of them in the wrecking yards and on eBay.