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I don't know much from your description. There's an awful lot of different "metal to metal" noises, like clank, clunk, grinding, squealing, pots & pans rattling, train wrecks etc. etc. etc. Some further info would help, maybe...

You say it has an exhaust brake. That's what slows the vehicle on a downshift. If it quits slowing on a downshift and freewheels, well, sounds like an exhaust brake failure. You didn't say which exhaust brake, or how it's actuated, like vacuum, hydraulic, pressurized air, mechanical, electric etc. Is it automatically actuated whenever the vehicle decelerates or what?

Whatever, my wild guess is the exhaust brake isn't holding its actuation, and re-actuates when clutch is depressed and released again.
 

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Yeah, good thought, or I mean, bad thought, well, whatever, Bob might be right! I'm thinking BD units call for a 60 lbs exhaust spring to prevent engine damage. That sound from too weak of exhaust springs would sound like the valves need adjustment. Could toast the engine...
 

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Some exhaust brakes use a bleed orifice [hole] in the shutter valve/butterfly to allow excess pressure to bypass, others rely on spring pressure at butterfly actuator. Jake brakes and some others also spec the use of 60 lbs exhaust valve springs. With a spring pressure bypass at butterfly, designed for a 6BT's exhaust pressures it might not work right on a 4BT, but theoretically the 4BT might not need that much bypass, hard to say. Whatever design is used it could be modified to allow more exhaust pressures to bypass by drilling one or a few holes in the butterfly.

I calculated the Pac Brake butterfly's bleed orifice [hole] at about 20% of total surface area of butterfly, but of course E-brake housing diameter has lots to do with it. I think Pac Brake is 4" housing, but not hard to find out by checking their website. Might also modify linkage so that butterfly doesn't close as tightly, causing more bypass.
 
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