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Have you checked the viscous fan clutch?. The noise seems external from the motor. So the noise appears only in gear and not in park and revved? You'll find it.
 

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A way to check the viscous fan clutch as Wraithman says is to remove the fan belts from the crank pulley and run the engine and see if the sound goes away.
The engine should be at operating temperature, and should not be run for more then 15 or 20 seconds, as the water pump will not be working.
Maybe one can just disconnect the fan clutch and leave the water pump still active, I'm not familiar with the belt arrangement on the car.
I realize this may be a huge job, not like a modern car where one can simply remove the serpentine belt by disengaging it from its tensioner.
If that very loud sound is coming from the fan clutch, just turning it by hand should reveal a bad bearing.
There should be no sideways play either.

What about exhaust, a lose heat shield or something internal in the exhaust, or even a damaged matrix in the cat (if there is one or more) ?
 

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It's also possible to have a broken baffle in a muffler housing that would rattle with higher flow when accelerated, although I think the noise would be a bit muted. You have to consider everything. A friend can help localize the noise.
 

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That doesn't sound like an internal engine noise to me either. One possible cause that has not been mentioned is the torque converter bolts. I have seen those loose on a couple cars and they will make a rattling sound. I had one Shadow towed into the shop a few years ago where two of the bolts had come loose and worked their way right out, catching the lower bellhousing dust shield and dragging it into the space between the torque converter and starter crumpling it up into that space. It made a horrendous sound. I would check those and if all OK there go to the fan belts next one by one, testing each time one is removed until you can test with all off. When you rev the engine, does it torque over to one side a lot indicating soft engine mounts? Probably not the issue because the fan hitting the rad shroud would be pretty easy to see and hear. As Rob says it could also be a loose baffle in a muffler or a bad exhaust leak, your recording sounds more like that rather than an internal engine rattle.
 

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Discussion Starter · #26 ·
Thanks guys, more to check it seems, especially external to the engine. I’ll have a look in the morning then away for a couple of weeks, need to get to the bottom of this to earn an early Xmas present !
 

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I was going to mention the flexplate/starter ring-to-torque converter bolts as well like Jim mentioned since this exact thing happened to me on the highway with my Suburban a long time ago. After removing the inspection plate a beat up short bolt dropped out. It will make quite a racquet and shock the hell out of you when it happens and it has no distinct pattern to it.
 

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Discussion Starter · #29 ·
Ok, checked the viscous coupling seems fine, resistance when rotating by hand, it’s not close to anything so it’s not the fan that is hitting something to create the rattle. Pic shows my set up. Plenty of space.

is there an inspection plate to see the flex plate to the torque converter bolts? Or is it a gearbox out job? May get round to removing the belts to prove its not an ancillary item.
Exhaust isn’t blowing.
thanks.
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Discussion Starter · #30 ·
Also front engine mount is new, engine does rock a bit when revved but it’s not terrible. I can see how this would cause interference issues with the cooling fan but I have no radiator shroud for that to hit. So yes perhaps the two rear mounts should be changed out but rocking of the engine does not appear to be the cause here.

I have listened to a couple of examples of noise from loose torque converter bolts on YouTube and it seems a similar noise to what I have.
 

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The belts that go on the power steering pulleys and air con pulleys look frayed on the edges. It's probably an optical illusion due to the angle of the photo but the air con unit seems to be set back slightly as the belts appear to be meeting the air con pulleys slightly off centre.
 

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Discussion Starter · #36 ·
Hi, belts are parallel, good spot re the two upper bolts but one doesn’t locate onto anything. Afraid I am unravelling 54years of other peoples work on the car. Removing the belts will help narrow down the issue. And yes it could be a cracked flexiplate. 😕
 

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Discussion Starter · #38 ·
Ok finally managed to get at the flexplate to inspect that. All bolts (3 off) are tight, no cracking around the bolt holes but of course I can’t view where the flexplate bolts to the crank.
Also removed belts and tested and noise still present. So it’s looking like I have to get the gearbox out to inspect the plate fully. I am also thinking the torque converter may be the culprit. Car has 143k miles so it could be failing - especially as it’s a load related noise and the converter is a load related device.
final test I want to do is a stall test with the brakes applied and the the throttle open, see if the noise is there then.
I intend to get the car into my garage, to take out the gearbox, 3ft of the car will have to stick out of the garage. I have some 4 tonne axle stands to get the car up in the air, and a transmission cradle for my jack. Any advice on dropping the gearbox? I have to remove the gearbox anyway to resolve a bad rear crankshaft oil leak with an upgraded seal modification. Maybe that leak is a clue that something isn’t right in that area. Some pics of the flexplate bolts too.
link to the procedure I will copy for gearbox removal although it’s for a spirit. Australian RR Forums: Gearbox overhaul SZ 3-speed



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Your smartphone camera is your friend. Also, witness marks, especially on rotating components that come apart...paint pen is helpfull
You will have to support the motor via the sump pan which is robust, blocks of wood will work well here. The torque conver houses 2 opposing fans and holds lots of fluid. When separating the gearbox from the engine, thread a heavy piece of baling wire across the torque converter and fasten so the TC will not slip off....they are heavy with fluid!
 
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