First--to answer another poster's note--the B18 has the identical timing gear parts as a B20. It's been my experience that a failing timing gear (in that the bond between the fiber gear and its hub is failing) has a sort of hollow cluk,cluk cluk sound--something between a rod knock and a main bearing knock and visual evidence in that when the timing cover is removed there are oil stain streaks spreading outward from the hub/fiber interface. There not much, if anything for the gear to hit within the cover--unless the gear is like crazy loose on the hub and striking the inside of the cover. On gears I've removed due to "knocking", the gear cannot really be moved relative to the hub - the bond failure "knock" happens early in the failure. The installation of a new gear and the subsequent disappearance of the knock has confirmed the diagnosis of a bad gear.
Maybe you can pinpoint the noise by using a stethoscope - or solid bar like a long screwdriver or socket extension and having an assistant slip the clutch out (with the handbrake on and the wheels chocked--or the car's bumper up against an immovable object--wall, tree, telephone pole, etc.) so there's a load on the motor.
As for the possibility of a bad motor mount--I've seen many of the stamped steel motor to mount brackets cracked--which is why I've kept a bucket of them. -- Dave