Troubleshoot slight brake clunk (2 Viewers)

[Super77]

I’m feeling some possible slop in the driveline that I’m trying to diagnose - wondering whether this is a common issue and where I should look:

From a full stop (like at a red light) and already in D, I occasionally feel a soft clunk when accelerating. There is also a light vibration at highway speed that seems to vary with how much gas pedal I give it.

Based on this thread, my gut tells me it’s either a U joint or something with the trans. I’m unaware if/when the trans fluid was flushed, but the fill level and color/odor are good. 142K miles.

Where do I troubleshoot first to try to get ahead of a possible future problem? Does this sound like a known issue?

 

[Trunk Monkey]

Tons of clunk threads, look at u-joints, DS grease, and drive flanges as most common culprits.

 

[Super77]

I’m revising this after realizing it’s probably not the driveline. When I release the brake at a green light, there is a very slight delay between when the brake pedal is fully released, and a soft “thump” that feels like it might be the caliper releasing when forward motion starts. Feels like it’s coming from behind me. It’s maybe 1/10 of a second delay, just enough to be noticeable, and only when accelerating from a dead stop. But acceleration has nothing to do with it, so I don’t think it’s the driveline.

Sticky caliper? Other thoughts?

 

[hotdawwgman]

I think I am experiencing this as well. I think it may be a sticking caliper.

 

[SteveH]

Feels like it’s coming from behind me. It’s maybe 1/10 of a second delay, just enough to be noticeable, and only when accelerating from a dead stop.

On Lexus GX470s (and many Ford F250s), this is stick-slip of the driveshaft splines. Any time the driveshaft splines slide in/out, they briefly stick and then slip and you feel the clunk. My GX had a new driveshaft installed by the dealer under warranty for this, and of course, the replacement shaft now does it, too. My LX470 does it on occasion, but not enough to bother me.

This super-light drive shaft spline grease worked on my Ford and my brother used it on his GX (he took the splines apart, cleaned them, and applied this grease) and solved the problem: https://www.amazon.com/Genuine-Ford-Fluid-XG-8-Lubricant/dp/B000NU77DA/ref=sr_1_3_mod_primary_new?crid=3CI9P7BQI79UH&dib=eyJ2IjoiMSJ9.Uv_mSvLMx6sQlPG5Q4ve3aPW6-20uGhAQgFkZPnFRMI1L7sxcb_QQyDHA5caCC3MQzj_Ie006SfIsIgAPLABr_vs8CjcBxW64stpQJgF1kdNBuz4b2nj5Z8JvzENOWVAvc2Ve4pvK8p1QnrfLK_krliPPp6NUbc19LyWUK1MqLRlTTgEaRythAp7jv4YXvuTGUlBxGYhBBtZVBJj0oiOkauJFH5_tO7qyAZ_KEEylC0FeEkbfBd8rxoU4CCqz5LH3GgTOA28JoyWE16LepmM0cNL1hIriT1UZKc58Rx-i3U.zadfZvgbZhTVnCUNwqQ8Pi1msujxDeALDWIkLzyIj8Q&dib_tag=se&keywords=ford+drive+shaft+spline+grease&qid=1745445666&sbo=RZvfv//HxDF+O5021pAnSA==&sprefix=ford+drive+shaft+spline+grease,aps,131&sr=8-3&th=1

I wish it were sold in a grease-gun tube, but it's not.

 

[Kamran Khan]

I’m revising this after realizing it’s probably not the driveline. When I release the brake at a green light, there is a very slight delay between when the brake pedal is fully released, and a soft “thump” that feels like it might be the caliper releasing when forward motion starts. Feels like it’s coming from behind me. It’s maybe 1/10 of a second delay, just enough to be noticeable, and only when accelerating from a dead stop. But acceleration has nothing to do with it, so I don’t think it’s the driveline.

Sticky caliper? Other thoughts?

I had the same clunk noise coming from the front caliper whenever I depressed the brake pedal. Upon inspection I found anti rattle spring that holds the disc pads in place were missing. After ordering a new set of anti rattle springs, the clunking noise went away.

 

[Super77]

I should describe it more carefully. My brake pedal seems to have 2 stages: the first inch or so of travel where there is no braking and almost no resistance in the pedal, and then after about an inch, I feel resistance and braking begins and functions as expected. So the brake pedal has an inactive, then an active portion of travel.

When releasing the brake and accelerating, it feels like there’s a soft thump when the pedal comes back up through the active portion and enters the inactive. Then there is a second thump when the pedal hits its upper travel bump stop.

I’ve looked in the footwell but see nothing mechanical loose. I’m unfamiliar with the electric brake booster on these rigs - is there some take-up or other adjustment that might be related to this?