Loud Banging Noise

[Vince]

I am trying to decipher what is causing my car to every so often make a loud banging noise like metal hitting on metal.  It only happens when I brake to a stop, not while driving or just hitting the brakes to slow down.  It seems to be coming from the front of the car.  The car is a 1967 RS/SS with a 350, 2 speed Powerglide, and power drum brakes.  It has the original wheels with wheel covers.  I at first thought it was the transmission downshifting extra hard when I came to a stop; I had a transmission shop go thru the tranny.  They adjusted the bands and replaced some gaskets so it no longer leaks fluid; they could find nothing wrong with the tranny, and of course the loud banging did not occur while they had it.  I had a local shop go thru the brakes and they could find nothing wrong with them.  So now I am stumped.  What do you guys think? 

[69Z28-RS]

A loud banging likely results from something 'large/heavy' shifting positions when braking...  Check every large item underneath the front of your car for loosened bolts, etc...

[maroman]

Like engine and transmission mounts.

[BULLITT65]

Like engine and transmission mounts.

X2, that was my first thought.

[ZLP955]

I would have a close look at the subframe connection points to the body, either worn bushings or rusted subframe holes.

[vtfb68]

Also check the simple things if you have not already, loose battery, loose spare, ect..
  Good luck,
    VT

[L78racer]

check for fan blade hitting something.

[olddragracer]

Powerglide transmissions have a bad habit of clunking or banging on a two - one downshift when braking to a stop. The engine must be idling smoothly at the correct RPM producing 18" Hg vacuum and not less than 15' Hg vaccum. If the engine is modified (performance camshaft) that will cause lower vacuum and cause the clunk.  With engine producing the proper vacuum the next most common problem is incorrect band adjustment. The band adjusting screw must be turned down to 70 inch lbs. the backed off 4 full turns then locked in that position. Often after overhaul or service of transmission the band will be set at 3 turns and this will cause the clunk. If vacuum and band adjustment are correct and clunk persists you can try backing band off to 5 turns and if that solves the clunk then the band sevo return spring may have lost tension and needs replacing. Be sure to reset the band to 4 turns.  If the transmission is not your problem maybe this information will help another powerglide owner.

[olddragracer]

should say:    band apply servo and not "sevo"

[Vince]

Thank you all for your replies.  I will do more checking and let you know what I find. 

[Vince]

Based on what I have found at this point and a shop that I have taken it to before it seems to be a lower control or a-arm not doing its job properly.  I am not quite sure at this point though exactly what is happening with the control arm that is causing the banging noise.  I am going to research further.  Again thanks to all for your suggestions.  It helped in narrowing things down. 

[Mike S]

 I had a Grand Prix that made a large 'clunk' noise most during braking.....it turned out to be a badly worn upper control busing. Check the rubber surrounding the top control arm bushings on the cross shafts (aka: dog bones).

Mike

[Vince]

Mine turned out to be a lower control arm that had too much play in it.  Every so often it, under braking, would move causing the banging. 

[x66 714]

Glad to hear you found the problem. It could've been more serious than that....Joe