I think you've got a vacuum leak in your power brake booster and/or its check valve. When you step on the brakes, it creates an air leak into #6 cylinder, roughening the idle and creating engine shake which causes whatever is hitting to hit and make noise. Remove the brake booster vacuum hose from its nipple, plug the nipple, and see if your "noise when step on brake" doesn't go away. Then put a muffler in the brake booster vacuum line (you can snag one off a GM vehicle). Then replace the check valve and grommet. If that doesn't fix it, then replace the booster.

Take a harder look at your trans lines...very closely all the way from where they attach to the trans to where they attach to the radiator.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

UPDATE

Well, fortunately, my fears of it being a spun main were unfounded. My TC bolts were way loose (I was able to turn them with my fingers). So I turned the crank by hand and tightened them one by one through the starter opening on the block. Piece of cake, and absolutely ZERO noise now. No exhaust leak, no valve noise, no ticking/clunking when I press the brake pedal, nothing. Baby purrs like a kitten and runs smooth as silk. Now I can continue on to replacing my lug studs and bleeding my brakes good (the crap is dark brown, fricken disgusting), then it's on to the HEI conversion!

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Probably nothing to it but I would also check the bolts and nuts that mount the tranny to the engine. I've seen those work loose and your noise when applying the brake could possibly be related to that. Probably not but it doesn't hurt to check it.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

Will do. Hopefully either this weekend or Monday I'll get 'er up on a lift and check over everything real good while I'm working on the brakes.

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'74 Duster w/ HEI ignition, beat to snot suspension, A904, 8.25" 3.55 SG rear, still being tuned up and gets 17 MPG

Know how they always build a better idiot? That's me

Good luck; it sounds like you're making excellent progress.

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David Kight
'62 Valiant Signet, White
'98 Dodge Dakota
'06 Jeep Liberty

Growing older is unavoidable but growing up is strictly optional.

What I would suggest, is, get some blue locktight, and while you are under the car, remove the converter bolts one at a time, and put the locktight on them, and retighten.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

Good job finding and fixing the loose bolts!

I would still check out the brake booster and valve, though.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Thanks to this thread I was able to determine the loud noise I heard was coming from loose TC bolts. I torqued them down and the noise went away.

Thanks, I thought it was going to be much more serious than that!

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Bob Brady
Slanted 67 Barracuda