My 4.2 L automatic transmission F-150 has an annoying rattle/buzz that only happens at idle or low rpm. I replaced the transmission mount thinking that was the source, but that hasn't helped much. Now I think it has something to do with an L-shaped hanger/connector between the drivers side exhaust and the transmission mount area. The L-shaped item is box steel with rubber or elastomer innards. Any idea what to call this thing?

 

have you checked your cats to see what kind of condition that they are in or have you checked your heat shields.

 

Yes, I've banged on the catalytic converters, and fiddled with the heat shields while the engine idles and while it is off and neither seems to be the source of the rattle. If I lever that "L-shaped" (maybe it's more "T-shaped") bracket, the rattle gets much louder. I tried to remove the bracket when I had the crossmember off to replace the transmission mount, but the nut holding it just wouldn't let go even with a heavy hammer and a chisel encouraging it to move.

 

I went back to the Chilton repair manual for the 1999 F-150 and found a photograph which shows the view from underneath, looking up, of the transmission mount, and the brackets which attach the exhaust pipe to the transmission. The brackets seem to be sandwiched between the transmission body and the transmission mount, and they act almost as a "tray" for the transmission mount. The photo is actually for the manual transmission, while my pickup has an automatic transmission. If I can post the photo, which I have scanned, I'll do so when I learn how to do that!

 

A scanned photo of the brackets connecting the exhaust pipe beneath the transmission mount of the 1999 F-150 with manual transmission can be seen at https://www.f150online.com/galleries...cfm?gnum=10571

My transmission is automatic, and the bracket is only connected on the drivers side. It is as if the passenger side of the "tray-shaped" bracket in the photo has been removed, and there is only one clamp on the exhaust pipe.

 

I tried to take off the exhaust and catalytic converter support but couldn't get the nut to move even with a chisel and heavy hammer. So yesterday I brought the pickup to a local garage that does exhaust work. I told them about the noise and the nut and they went to work. They phoned back that they thought the mount was the problem, and would replace it if I wanted. I told them to go ahead. They rang later saying maybe it wasn't the mount, but there was some leakage from the automatic transmission extension housing and maybe that was the root cause because there was play. I knew about the leakage because the transmission mount I replaced was fouled with oil and I could see it was coming from the extension. I told them to go ahead and replace the seal and bushing.

This afternoon the pickup was ready and it doesn't buzz. They found the exhaust and catalytic converter support had a crack around one of its mounting holes, and I'd say that was the cause of the noise. The transmission doesn't leak now, so I'm happy. Hope the seal keeps sealing. I'll post a photo of the support in the galleries, and next posting should have its URL.

 

The photo of the cracked support is at:
https://www.f150online.com/galleries...TOKEN=75148986

I hope the crack and the text can be seen and read.

 

That last URL doesn't get to where I wanted to go. Try
https://www.f150online.com/galleries...cfm?gnum=10571

 

ford is notorious for this crawl down there and check your heat shields. if they're loose re weld them or rip them off, or even outright replace the cat. im guessing your hearing a high pitched ringing sound kinda like a bell?

 

No, the noise was more low-pitched, not musical at all. I found the cause, as indicated in the previous few postings.

 

i have this very same problem with the rattling/buzzing noise...i'll have to crawl under and check it out....thank god i am a welder...

 

I wonder if 5.4 trucks have this bracket too, I have a really good vibration in drive with the brakes on, not so bad in neutral and it is gone in park, it all started when I had duals put on. Mine is just a vibration and a drone though, not so much of a rattle.