There are at least two possibilities:
1. The case of your L640 is loose - try to tighten it.
2. The vibration motor assembly is loose - you would need to open your phone up and secure the loose vibration motor assembly (or simply have a service center do that for you).
If you have a video sample of the claimed increase in vibration intensity, we would be able to be more precise with our findings. There was a possibly similar issue reported in the forums about a month ago - the problem was that the vibration motor assembly was loose and placing a piece of plastic in between the assembly and the cavity secured the position of the motor.