Hey,

I've got an 04 F-250 W/ V10 and ZF S6-650 6spd manual transmission.

There is a consistent lower frequency knocking that increases with engine speed when the transmission is in nuetral and you're not pressing the clutch pedal . The noise goes away when the clutch is pressed, it is in any gear, engine speed is above 2K RPM or so, or, without pressing the clutch, you slightly press the shifter into any gear (without really trying to shift and consequently gringding of course)...

I've been told that it might be the input shaft bearing, but besides the knocking, there doesn't seem to be any kind of whiring noise or anything that I would normally associate with a bearing. I've been driving the truck hearing this noise for about a year and it does seem like it's gotten slightly
worse. That being said, I usually don't let it sit idling in nuetral as to avoid
the knocking noise and if I do want to let it idle for anything past a minute or so, I shift the transfer case to nuetral and then put the transmission in gear, once again avoiding the knocking noise. If I didn't do these things, I guess whatever the problem is would probably get worse much faster, but I am a procrastenator, so yeah..... Any ideas as to what exactly the problem could be? If I have at least a decent idea of what the problem is, I wouldn't be afraid to try to make the repair myself.....

 

Thats probably just the "gear rollover noise" that is pretty common on all these zf transmissions. Mine has done it ever since I got rid of the dual mass flywheel when I put a clutch in it. If I'm not mistaken the F450 and 550 came with the single mass flywheel from the factory. I dont know for sure if thats your problem without hearing it but if you search here or on some of the other diesel websites for gear rollover noise I'm sure you can find more info. If that's what it really is then it is just the way the transmission is designed and there is nothing to do or worry about. The noise comes from the slight backlash in the gear's teeth bouncing back and forth from the vibes from the engine's power pulses.

 

Tim:

This truck and my last truck, a 02 F-250 reg cab w/ 5.4 v8 and 6spd manual, both do what you're truck is doing and yeah, it's just backlash through all the wonderfully large gears these trucks have in them and is normal. Unfortunately, this is not what I'm describing... something in there is knocking around when it's not supposed to....
And correct me if I'm wrong, but gasoline engines only ever had single mass flywheels,
right? (Fords of these years, anyway...)

 

Yea sorry I did'nt catch the v10 part til after I posted. If the noise is going away when you put it into a gear then it sounds more like a needle bearing under a gear and not a main or input shaft bearing. If the trans is in neutral then the gear bearings are turning full speed. If the transmission is in any gear then the gears. Will turn at different speeds in relation to the mainshaft. Either way sounds like it'll need to be pulled to further diagnose. Also I would check the pilot and throwout brgs once its out though it doesn't sound like the symptoms are for one of those