Thumping noise when changing gears? - Impala Tech
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post #1 of 7 (permalink) Old 10-21-2015, 08:57 PM Thread Starter
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Thumping noise when changing gears?

When I change gears to D or R, I hear a thumping noise every 2 seconds. It's perfectly fine though when it's idling in park and even when I rev it. The noise only happens when I go to a different gear.

I had my Dad sit inside and have him change to R and D. First thing I noticed was the engine block rumbling rough (shaking every 2 seconds). I also bent down to get a look at the underbelly and I saw the drive train moving slightly back and forth clockwise and counter clockwise, corresponding with the thumping noise. My Dad also said while even in park something felt off (not enough power) - which is probably a totally unrelated problem.

Any ideas on what's going on? I'm probably going to get it towed to my mechanic anyway. It's still good to have an idea of what's going on first.

It's worth noting that it has been sitting for a while (like 2 months) and I didn't hear this noise before that. I also recently finished putting in new mufflers and this is basically one of the first start ups. The very start up after the two months and muffler install 2-3 weeks ago, it was fine in all gears.

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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post #2 of 7 (permalink) Old 10-21-2015, 09:16 PM
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Could be mufflers or exhaust pipes hitting under body due to engine movement. You may have a broken motor mount as that is very common malady with these old cars.

Big Dave
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post #3 of 7 (permalink) Old 10-21-2015, 09:21 PM Thread Starter
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Quote:
Originally Posted by Big Dave View Post
Could be mufflers or exhaust pipes hitting under body due to engine movement. You may have a broken motor mount as that is very common malady with these old cars.

Big Dave
I guess I'll check the pipes again. When putting the mufflers in I made sure there was nothing touching the body though.

Motor mount sounds logical. If not maybe a transmission mount?

Edit: Well Dave, I think you might be right about the exhaust. When harnessing in the new hangers (adjustable clamp hangers, thankfully), I may have ignored the H pipe section. I'll have to check it tomorrow but I may have accidentally harnessed everything in higher then it should be. Therefore, causing the H pipe to touch the drive train and cause them to grind against each other.

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350

Last edited by Classiccarman; 10-22-2015 at 02:24 AM.
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post #4 of 7 (permalink) Old 10-22-2015, 07:35 AM
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The rocking motion of the engine indicates a miss-fire and your Dad's observation of something "off".

The exhaust may still be too close to the floor or driveshaft, but I would be looking at the engine not running right also.

If you are burning regular, oxygenated gas, it may have separated after sitting for 2-3 weeks and has gummed up the carb. If the carb hasn't been rebuilt in the last 8-10 years, the alcohol in the gas deteriorates all the rubber parts and ruins them.

Only use non-oxy gas in a carburated engine.

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post #5 of 7 (permalink) Old 10-23-2015, 09:55 PM Thread Starter
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I went to check the firing order and it turns out two of the spark plug wires weren't on the right plugs. After figuring that out, I turned it back on and ran through all the gears with no problem. Test drive went great as well.

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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post #6 of 7 (permalink) Old 10-23-2015, 11:41 PM
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Quote:
Originally Posted by Classiccarman View Post
I went to check the firing order and it turns out two of the spark plug wires weren't on the right plugs. After figuring that out, I turned it back on and ran through all the gears with no problem. Test drive went great as well.
That will do it...were #5 and 7 crossed...

'62 Impala SS 409 TH350
'66 Chevelle SS 496 M20
'70 Chevelle SS 396 M20
'67 Camaro ss/rs 350 PG
'38 Chev coupe street rod
'54 Chev 210 2 door
'69 Chev C10
'89 Chev R3500 roll back
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post #7 of 7 (permalink) Old 10-25-2015, 02:43 PM Thread Starter
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Close guess. 5 and 2 were actually crossed. I have no idea how I managed that seeing how they're on opposite sides of the block. Must of happened when I was checking spark plugs. Human error!

1967 Chevrolet Impala 4 door "post" sedan
327 5.4L V8 - 3-speed TH350
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