Going to necro rez this thread to share my experience with an identical issue.
Couple weeks ago I noticed that once in a while the vibration motor of my Evo 4G LTE (bought in June 2012) would turn on for a second or so for no particular reason (no incoming notifications). Several days later the motor started vibrating constantly, even when the phone was powered down. Taking off the back cover and "massaging" the area around the camera LED and headphone connector would slow down the rotation speed and sometimes completely stop it, but it would always start rotating again in minutes/hours. Putting the back cover on would always start the motor again.
I took the phone to the Sprint store for repair. The tech said he had never seen this issue before, but was quickly able to figure out that there was a short circuit in the two-pin connector for the vibration motor leads. Luckily, he had an extra connector, so I was out of the store in about 15 minutes with a perfectly working phone. Who knew I didn't have to RUU and S-ON, the guy didn't even turn it on
HTC Evo 4G LTE
Baseband 188.8.131.520 | PRI 2.97_003 | TP 3.2 | PRL 56013
SecureFlag S-OFF | HBoot DirtyRacun 2.09.2222 | Recovery TWRP 184.108.40.206
Kernel ElementalX v4.1 | ROM MeanBean v3.09 | Theme BLS v2.5
Case Otterbox Defender | Protector Steinheil UC
ASUS TF700T 64GB
Bootloader 1.00e | Recovery TWRP 220.127.116.11
Kernel Hundsbuah-Data2SD v2.1.0 | ROM CROMI v3.4.6 | Theme CTM v1.4
Case Gumdrop Drop Tech | Protector Acase AcaseView Clear | Dock TF201
MicroSDs SanDisk Ultra 64GB Class 10