if the sound goes away when you push the clutch pedal in then it's your clutch release bearing. Even a little pressure on the pedal will make the sound go away.
Actually - in my experience - if the sound goes away when you push the clutch in, the clutch release bearing is the last thing it's likely to be - assuming a properly adjusted clutch - I'll explain this later..
If the vehicle is stationary with the engine idling, when the clutch pedal is released (clutch is engaged), the transmission input shaft and depending on the transmission design the countershaft or layshaft will be rotating, the output shaft will be stationary - any one of several bearings supporting the input shaft and countershaft could be making the noise, and you may be able to pinpoint which bearing it is with surprising accuracy, by listing to the noise with the vehicle in motion and observing which gear it is loudest in.
If it is a bearing in the transmission I would suggest a transmission rebuild job and change out all of the support bearings and seals - by support bearings I mean those that support the gearshafts in the casing, rather than the needle bearings on which the gears themselves rotate.
Back to the clutch release bearing - if the clutch is properly adjusted the clutch release bearing is not rotating with the clutch pedal is released and so should not be making any grinding noises - as you depress the pedal the clutch fork pushes the bearing toward the flywheel until it comes in to contact with the fingers on the clutch plate and then depresses the fingers causing the clutch plate to lift so that the clutch disengages and the engine continues to turn, without turning the transmission input shaft. This is the point at which the release bearing is rotating - the outer part of the bearing is in contact with the clutch plate and rotating, the inner part of the bearing is on the clutch fork and stationary - this is when a defective release bearing makes noise.
If the noise occurs only when the clutch pedal is depressed (with the engine running) that would be a defective clutch release bearing.
It's also possible for a bad pilot shaft bearing (the little one in the back of the flywheel - also known as a spigot bearing), to make a noise when the clutch pedal is depressed, but this is not as consistent as the clutch release bearing, since it will depend on which gear you're in and if the vehicle is moving - both of these affect the rotational speeds of the two parts the flyheel that the outer part of the bearing is attached to, and the pilot shaft, that the inner race of the bearing is attached to.
