Hey guys, I love this forum and come here all the time to get help with questions, but I haven't posted a ton yet. I've been doing a little research but haven't found a real definite answer for my current situation yet so I thought I would see what you thought.

So I've got a 79 f150 351m 4 speed. Its been working pretty well for quite some time but just recently has developed a sort of "thud" in the drive line somewhere. It's not a super big noise, but it has me worried.

Here's what happens.

-during acceleration I can make it thud. It usually happens when the clutch is just more than halfway out, but I have gotten it to do it after the clutch is out when I accelerate more. I

-I have gotten it to do it in 1st, 2nd, and 3rd. It only happens once during acceleration, so I don't think its transmission related, but...?

-If I'm really gentle on acceleration, it is minimized and sometimes almost gone.

-It's not a super loud or heavy noise, but I can feel it through the gear shift. It almost feels as though the transmission is shifting under the torque of acceleration.

-In addition to this (i don't know if its related or not) sometimes I will hear a light almost clicking noise matching my wheel speed.

I am sort of thinking it may be the rear diff bearings or something? Could it be a transmission mount? I don't think its the clutch, but I really don't know for sure.

Anyway, I am currently going to school and live in an apartment so it's kind of difficult to get the rear end up on jack stands like I would want too and look at the drive line, but I could go get some stands from Harbor Freight if needed. This is my DD and I was planning on going on a trip 3 hours away this weekend but am wondering if I shouldn't drive it until I figure this out?

Any ideas/comments/help/knowledge would be super helpful and greatly appreciated! Thanks!

 

Since the thud can seemingly be manipulated, check for the hardware integrity.. ..engine, trans, t-case bolts and their mounting points.

Also check if any of the mounts' insulators have separated.. you'll need a jack and a piece of lumber. They may look OK but only reveal their failure when in use.

 

As for the wheel speed clicking. At slow speeds it can happen if the front disc brake pads don't have the spring clips in them correctly. The pad grabs the disc and falls repeatedly at slow speed.

Hard to diagnose thuds over the internet. Just have to investigate the hard way most of the time.

 

Best thing you can possibly do is have someone in the cab that you trust to put in drive and reverse with brake mashed. Then just observe...

Could be rear end slop, could be one of the u-joints, could be the center axle bearing, could even be in the transmission...

But I'll bet it will be obvious!

 

my money is on u-joints

 

mine's on the rear end backlash or spiders

 

Ok, here's the update. I drove it around some more and then climbed underneath it and I've definitely determined that there is play in the rear pinion. It's not a ton, but its definitely noticeable and I'm pretty sure its the problem.

So, here's my next question. Would it be totally stupid to try and drive this thing 400 miles this weekend? I was really planning on making a trip for labor day, but now I'm wondering if my chances of destroying my rear end are really high. Or can I drive on it for a little while still? I would like to fix it, but I don't have enough time before I was planning to leave.

 

If the bearings are going out in the pinion it can take out the gears pretty quick. Happened to my 78 with a 9" rear. Bearings were loose and the PO had driven it that way and wore the gears really sharp. Had to put all new bearings ring and pinion in it.

 

It could be a worn out bushing/slip joint in the back of the tranny that sticks.

 

I drove my truck for over 50,000 miles with a broken carrier in my 9 inch. It was sloppy when I got the truck so who knows how many miles it ran like that. I was in college and didn't have the time money or facilities to fix it.

It was the trac lock carrier which is prone to breaking. Mine was cracked all the way around and through the ring gear bolts. It had probably 45 degrees of slop in rotation at the pinion.