1996 Ford Aerostar a noise and vibrations right before shif

I am looking to buy this 96 aerostar it has only 70k and is in goood condition everything seems to point that miileage is correct. When test driving it it drove great and shifted good but when I stepped on it. It drooped down into passing gear fine but right before it shiftes into next gear there was a roaring sound then a vibration then it shifts and all is well. The roaring and vibration are for a short time and only show up then. The others that I have driven did not do this but they were a 4.0 this is a 3.0. Trans? Harmonic balancer? Rpm. Not mph. Seems to cause it it has been used very little in last 3years it has a 3.0 motor