350 ci Chevy vibration

I developed a vibration in the motor that has me puzzled. At first I thought it was a u-joint as it had a rhythem to it.  Then I noticed that setting with the tranny in park and reving the motor to about 1500rpm or so the same vibration was there.  At idle it is normal.  As the rpm increases the vibration stays about the same. The tranny is a 350. Any ideas?  

You might start by checking harmonic damper on front of crankshaft on engine. Sometimes the fail when rubber laminating in them goes bad.  

Definately check harmonic balancer but due to internal balancing it would have to almost be falling off to cause vibration, also check motor mounts, it is often overlooked but the engine always vibrates we just don't feel it because of the rubber mounts.  

The fact that it is RPM sensitive tends to suggest a balance issue. While a 350 is internally balanced it still needs the balancer to tune out harmonics that can occur.  

The harmonic balancer is new, do you think it could be bad from the factory?  I suppose anything is possible, but I spent a little more and put a fancy dandy one on to make sure things were right.  Thanks for your input.  

Did problem start after balancer was replace or before?  

it may be the wrong balancer it defiantly seems like a balancer issue to me. it many be a fly wheel issue pull  the trans inspection cover of and check it it may be cracked or broken. i have seen fly wheels cause the same problem. good luck  

The motor was repuilt and has been driven a total of about 100 miles.  I noticed the viberation first when driving and thought it was a u-joint.  One day I started it and after a short warm up I brought the tack up to about 1200 or so and noticed the viberation again.  How can you tell if it is the wrong balancer or not?  It is a after market one that was bought at the local speed shop. I will pull the inspection cover and check the fly wheel out.  thanks  

Get a new OEM/stock damper. The one on there is likey the wrong one.