If you subscribe to the free "HELIcopter PROfessional Pilots Safety Program" (HELIPROPS), there is a good article on page 7 about vibrations caused by icing in a UH-1. To summarize, while in IMC the ship experienced lateral vibrations that would build in intensity and then they would feel a jolt with no more vibrations. These events would repeat several times. Apparently, they were taking on ice and then shedding it off.
Although sponsored by Bell, the articles are focused on helicopter safety and include many other types (MD, Robinson, etc). It's free, it's pretty interesting reading, and you'll receive them about once a month or so...
Bell Helicopter Textron, Inc.
Jim Szymanski, HELIPROPS Manager
P.O. Box 482
Fort Worth, TX 76101
OR
Fax (817) 278-2428
OR
[email protected]