grinding noise
 

i have a 2001chrysler voyager, i just replaced the motor 6000 miles ago runs great. yesterday it stared making noise, i went to atart it and it sounded like the start stayed ingaged but not i turn it off soon as it started making noise the restarted the third time, driving down the road ti started making the noise agin it almost like if a gear was bing stripped out, i reved the engine and it went away, i dont think it is the trans or the starter and i check the bolts on the flex plate they were tight, but some thing to do with the rpms, when it makes the noise i see some fluxation in the rpms and when the engine is reved it go's away any help please, thanks