Soundless vibration is an irritating thing; it can signal the onset or manifestation of a problem within the engine itself, or it could just be a little glitch in some ancillary mechanism. But it is hard to say, so you'll need to engage in some proper diagnosis before laying down a penny for repairs.
Broken Cable Brackets
The gas pedals in most modern cars connects to the engine via a cable, which rides inside of a sheath mounted to the throttle bracket. When you press the gas pedal, the cable moves inside of its sleeve and pulls the throttle blade open. Under normal circumstances, vibration in the engine will never reach the pedal because the whole sleeve moves with the engine. If, however, your throttle cable bracket is cracked, broken or loose, the bracket will -- to some extent -- move independently of the engine. This will cause the cable to move in and out of the sleeve, transmitting vibration to the pedal.
This problem is similar to the broken bracket one, but involves the cable itself. Most throttle cables have a threaded bit on the end that allows you to make adjustments by moving the cable sleeve closer to or further from the engine. A loose adjustment collar allows the cable sleeve to move independently of the bracket, resulting in sloppy accelerator response and possible vibration in the accelerator pedal itself.
While the throttle cable generally does a good job of isolating large vibrations, minor, high-frequency vibrations can still work their way through the taut cable and to you pedal. You engine's harmonic balancer and flywheel will kill most of these vibrations, but they can't deal with stuff that's not attached to them. Bad bearings in your alternator, power steering pump and AC compressor can cause vibrations, but will also usually cause some sort of noise. Loose accessory brackets will allow the accessories to vibrate back and forth, and a badly bent or damaged cooling fan blade with oscillate and vibrate at certain speeds
Electronic Throttle Problems
Many modern cars incorporate some kind or electric throttle or "drive by wire" system as part of their traction-control strategy. The electric servo that controls accelerator pedal feedback can malfunction; or, more accurately, the position sensor in the servo can malfunction. If the servo loses position feedback, or if something goes wrong in the system, the feedback motor may end up "searching" for the correct setting to stay within the program parameters. In practical terms, this can mean an odd vibration that occurs randomly, or when you do something like engage the headlights or power windows.