Odd vibration at speed... any ideas what it could be?

Hey Guys,
The last few times I've had my bone-stock 75 Swinger on the highway, cruising over 65, there is a slow pulse-like vibration that I feel in the front end and steering wheel.
It's about the speed of a slow heartbeat, but each pulse is a series of very fast vibrations. It's not a violent shaking of the wheel by any means. At first I thought I was imagining it, but it seems distinct and if I didn't know any better I would think it's getting worse.
Any ideas?
Thank you!