At 40 mph the throttle pedal on my 98 Chevy Silverado vibrates. What is wrong?

try clamping down your throttle cable with a couple of cable ties. Sometimes the factory clamps come loose and break with the heat. Also does anything else vibrate or just your throttle. IS your truck 4X4?

Nothing else vibrates. It is not a 4X4. Where would I clamp down my throttle cable at?

under the hood just follow it back from the intake and clamp it along the way to existing brackets. You're sure its not the torque convertor shuddering?

I don't have any idea what it is, but it is consistent.

the next time it does it just tap the brake pedal and see if goes away for a second.

I will do it on the way home tonight. Thanks