strange pulsing sound

[4404spdcharger]

My 1968 440 Charger started doing something weird lately.  Between 45-50 mph, there is a pulsating sensation that disappears if I go faster or slower than the 45-50 mph.  I've had the front end aligned, just put on some new Coker redlines and balanced them, It's an unrestored car in pretty good shape but this is annoying.  When I push in the clutch, the problem goes away so it must be drivetrain related.  The engine mounts are tight, I have been under the car on jacks and nothing is loose, broken, or bent.  Any ideas???

[John_Kunkel]

Goes away when the clutch disengages the motor from the drivetrain? More likely it's engine related such as a lean surge. Does the pulsating occur at 45-50 in second gear or only 3rd?

[Brock Lee]

I don't have a clutch, but my car did that as well. My driveshaft was out of balance as a weight that once was on there (Factory?) fell off. It would react around 45-50 MPH. It did a pulsing that actually made a vibration you could feel. But it only acted up in a narrow band of MPH. Above and below and didn't do it.

[Ghoste]

That you would still feel if you had a clutch though to disengage though.  John is right.

[4404spdcharger]

It's nearly always in 4th gear.  I had the engine worked on last year and they dyno'd the engine.  The tech said he changed the rods in the carb and now it runs the best ever.  No hesitation off the line.

I checked the universal joints and they are good.  I didn't think about the drive shaft (duh) but maybe I should pull it off and get it checked.  Thanks, guys.

[six-tee-nine]

If it goes away when you push the clutch then it should be engine related. That could mean your problem is related to a certain engine speed an not driving speed.
you shoud try to find out if the same sound occurs when running f.i. in second gear and bring the engine up to the same revs as in 4th when doing that 45mph

[JR]

Mine used to do that.

In my case it was a slight bend in a rear wheel. I replaced the bent wheel and it all went away.

Also, are the balance weights still attached to the driveshaft? If they've fallen off, I could see the same thing happening.