RudeXJ

I'm gonna try keep this as simple as possible.

I have a phantom knock inside my Jeep (4.0 2000 145 miles on the clock) that I can only hear in the cockpit. It occurs after I've been driving a few miles and is only audible at idle and speeds up momentarily when I accelerate and then disappears. I've tried to listen underneath and in the engine bay and can't really pick up anything and I've used a mechanics stethoscope, no luck.
It doesn't seem to effect performance at all, but is seriously bugging me!

Its none of the usuals as I've already checked/replaced most of them when I had other issues:
Not lifters, did a whole head job recently as I had a tick and thats sorted.
Not piston slap, I have it faintly and is totally not the same sound.
Definitely not flex plate, checked that twice now and its all good
Not trans case lever - it does clatter sometimes, but not the same noise.

Sounds like its coming from in the dash...I'm stumped!

Check out link to vid here (turn up the volume)

winston427

The torque converter bolts are notorious for backing out and making contact with the transmission bell housing. Very common problem on the XJs. This is exactly what it sounds like to me.

RudeXJ

which is why I checked it twice...properly.
I know it makes the most amount of sense, but the bolts are tight and there are no cracks, unless there could be another flexplate issue that I've overlooked?

jordan96xj

Have you ruled out exhaust clearance over the cross-member? Typically due to a sagging/old transmission mount. That often has a way of being noticeable at idle and early acceleration, but fades as the speed picks up and torque load decreases.

cruiser54

Great idea. Along with motor mount issues.

RudeXJ

I'll check that out and report back, thanks!

fb97xj1

Block the wheels and set the brake, and give it some gas in neutral. Rule out the drivetrain atleast. Remove the belt and try again, to rule out an accessory, if the noise is still there.

RudeXJ

Thats a great idea!
Ive replaced rear U joints, need to do front, but I don't think that's it as I stuck my head under when it was knocking and front shaft was still and couldn't hear a thing. (ps not the exhaust or tranny mounts, they good)

i have a knock/clatter that sounds similar, but checked it out, was the trans lever loose in its bushings apparently common and no biggie...I'm hoping this is similar and not a transmission problem...please dear god.

will post a longer vid with a complete walk around at idle tomorrow. Cheers for the help so far.

Superslow

One word, ok three words: Evap purge solenoid. Put your stethoscope on that sucker. It attaches to the firewall on the passenger side with an isolation bushing.

RudeXJ

Ok guys so I pulled over today when the knock was at its loudest, I had my stethoscope handy and prodded everywhere; Its not the Evap purge solenoid, although I do seem to have a slight vacuum leak...joy.
I prodded all around the firewall and underneath and absolutely no trace...but inside its still knocking away.
Is there anything IN the dash that could be causing it, also my a/c doesnt work, could it have something to do with that?

cruiser54

How about the exhaust pipe knocking on the cross member?

RudeXJ

Hey guys, so I have an update;

I was underneath the jeep to remove the front drive shaft in order to replace u joints and when I wiggled it there was a distinct knocking noise that could be the culprit.
When I put my hand on the transfer case I can feel the vibration on the other side of the front axle output flange.
I've removed the shaft and its not the u joints knocking, they're solid, the cardan joint sounds bone dry however (can I just regrease it or do I need another one?)
A crappy vid below, but gives you an idea, camera is pointed at the transfer case, from underneath whist I wiggle the prop.


Dry Cardan Joint culprit?


Also I've driven it briefly and since the front shaft is out, there seems to be no noise, but yet to confirm...If I wiggle the t-case front output flange without the shaft, the noise is still there, albeit not as severe.

RudeXJ

So I drove it today without the front shaft. The noise is still there, but seems at about 50% to what is was. Gonna put my stethoscope on the t-case tomorrow when the noise starts again. Anyone know what is causing that play in the t-case output, worth worrying about?

s346k

if it's knocking when stopped i highly doubt it is a drivetrain part. assuming the flexplate/converter has been ruled out. have you done a compression test since this started? changed the oil and inspected the old stuff?

RudeXJ

It's intermittent, I assume it's the idle vibration at stop causing the knocking and as I accelerate it increases and eventually as it gets up to speed functions without play - or that's my reasoning anyway, gonna stethoscope it tomorrow to confirm.

Yep ive ruled out engine knock (I've had that) flex bolts, checked twice, changed all fluids in the last month, no metal in any of them.

Its none of the common things, honestly this is my only lead.