Each bank has two cams. Only one of the two cams on each bank is driven from the timing belt. On the front bank, the exhaust camshaft is driven by the timing belt. On the rear bank, it's the intake cam that's driven by the belt.
The "other" cam on each bank (i.e. intake on the front bank, exhaust on the rear) is driven by the first cam through helical gears (you can see them if you take the oil cap off and look inside the cam cover...)
The helical gears have backlash and can be quite noisy because of that. The "friction gear" is designed to pre-load the gears and take away noise-causing backlash.
The friction gear bolts to the "driven" cam on each bank. It looks like a skinny gear right beside the main gear. It's held in place by a large nut on the end of the cam and is preloaded by a large, springy shallow-cone shaped washer called the "friction gear spring". It rides right up against the driven gear.
So it rubs against the driven gear and meshes with the drive gear. Here's the thing: this gear has one more tooth than the driven & drive gears, so it turns at a slightly different rate than the driven gear. Because of this, the driven gear and the friction gear rub together (hence the term "friction"). Because both gears are driven from the same drive gear, but turn at different rates, the system is "loaded up" and the backlash is taken out.
Friction gear springs can wear out (relax) over time, causing the friction gear to free-wheel. (IIRC, there was also a design change for the 1995 year that addressed the friction gear spring itself. When I had my springs done, they installed an updated spring on each bank.) When this happens, the lash is not taken up and the drive gears get quite noisy and clattery, especially because the action of driving a camshaft is not a smooth one, with 6 springs & lobes all trying to stop the shaft at "uneven" times.
The clattering noise from bad friction gear springs generally quiets down after about 2000RPM, but as the engine RPM comes back down, it will re-appear again. Mine sometimes makes a clattery noise when it's cold out (damn Canadian winters) but it usually goes away after a few minutes. Oddly, most mornings I never hear it and others it's quite bad...inconsistent engine
