Rattling Sound Around Flex Plate: What Is It? HELP!

[colnago]

My truck (352FE w/ C6 tranny) just started making a rattling sound around the flex plate. I took a stethoscope to it, and it is definitely in this area. It dramatically drops off as I move away just a couple of inches. I checked the four mounting bolts to the torque converter, and they all were tight. I also tapped the flex plate with a wrench, and didn't find any spots where it buzzed.

While idling, it rattles noticeably. If I leave it in Park and rev the engine, it still rattles. If I put it in Drive, then it stops rattling as long as I'm accelerating or decelerating. But once I level off at a constant speed, it starts rattling again.

Does this sound like a flex plate? Could it be the torque converter instead? It sounds like something is loose, but I sure can't find anything. If it is either the flex plate or the torque converter, can I damage anything if I keep using the truck to commute in-town? Can I replace the flex plate in my driveway without pulling the engine, or dropping the tranny?

Thanks,

Joseph

[averagef250]

I would start by seeing if parking on a hill nose up or nose down while running makes a difference in the noise. If it does it could be the crank thrust bearing. I only say this first because I ran into this once and it was an ordeal to troubleshoot because I wasn't looking in the right place. One time I parked the truck idling on a down hill and the noise was gone. Pointed uphill and it came back. Thrust bearing was in the pan.

If that checks out my WAG would be the torque converter has lost a bearing or bushing inside and needs to be replaced. Could be cracked flexplate, but they usually fail pretty fast once they crack- You don't get weeks of driving when the center tears out.

[Donnie]

Hello, yes, Average does have a good point about the engine thrust bearing....So since it is easy to check the engine end play..Start there.
But since the torque converter has no bearings, only thrust surface washers on either side of the Stator that are aluminum in the early C6 & Bakelite in the later C6's...it doesn't sound like what you are describing........If these thrust washers wear enough to allow the converter pump,(rear 1/2) really called the impeller to contact the turbine, you would hear the fins contacting each other. ZINGING is how I would describe it...LOUDEST when sitting still, engine running & the trans in any gear......NOT MOVING ! ! ..any gear stops the turbine from spinning and if the impeller is making contact with it you will hear it now...But the sound will be "Muted" as the converter is full of oel...
My best guess is that your flexplate is starting to crack & it will take out the center soon & do a number on your front pump..
Why not disconnect the converter nuts & push the converter into the trans an 1/8 inch....in other words loosen it from the conv.
Now with your choice of tapping tool's listen to the flexplate for an unusual noise..You may be able to tell if it has a crack..
that's my best shot...........good luck & let us know.........Donnie

[colnago]

Right now, the plan is to have the local tranny shop take a look at it tomorrow. I stopped by today, and the "ear diagnosis" agreed that it was related to the flex plate. He's going to check the bolts for correct tightness, and for cracks, then go from there. I figure that if it's bolts, then it will be fairly inexpensive for the shop to snug down; if it's more than that, then I couldn't do it on my own anyway. More news tomorrow.

Thanks,

Joseph

[colnago]

Well, the tranny shop tells me that it's a cracked flex plate. I would have picked her up today, but the shop got the wrong replacement flex plate shipped to them (supplier had the wrong plate in the box).

Ah, well, this is the first real money I've put into keeping the old girl on the road. I guess I can't be too surprised that a flex plate can't last forever (although it picked a really stink-o time to go south on me).

Thanks for everyone's input and interest,

Joseph

[Donnie]

Joseph, nice move , catching it QUICK while it was only cracked...If the center had broken completely the converter would have gotten loose & taken your fr. pump & possibly the converter with it........fast response = ....................donnie

[colnago]

Thanks, Donnie!

Interesting side note. I picked her up today, and they said that the flex plate for a 352 didn't fit. The shop said that I probably have a 360 or 390. Huh? Do the bigger engines have a different bolt circle on the crankshaft for a different flex plate? I know the old girl was born with a 352, and I thought she still had it. I guess I do the dowel-in-the-spark-plug-hole-to-measure-the-stroke trick this weekend. Any easy way to measure the bore without popping the heads?

Dang! I have a 352 on the garage floor waiting to be rebuilt. Now I don't know what to do!

Joseph

[averagef250]

Thanks, Donnie!

Interesting side note. I picked her up today, and they said that the flex plate for a 352 didn't fit. The shop said that I probably have a 360 or 390. Huh? Do the bigger engines have a different bolt circle on the crankshaft for a different flex plate? I know the old girl was born with a 352, and I thought she still had it. I guess I do the dowel-in-the-spark-plug-hole-to-measure-the-stroke trick this weekend. Any easy way to measure the bore without popping the heads?

Dang! I have a 352 on the garage floor waiting to be rebuilt. Now I don't know what to do!

Joseph

They all use the same crank bolt pattern. Same as 429/460 too.

Nice that you found the problem without it becoming a big problem. Hopefully they didn't charge you too much.