In increasing order of severity:

1) Look for loose components rattling on the engine. Loose brackets, busted motor mounts, etc.

2) Valve train clatter - loose valves tap tap tap (a fairly light weight noise) a little louder than normal. Usually valves tend to tightten slightly over time given plenty of neglect, however. And tight valves burn themselves. So an unusual lack of clicking is actually a warning sign. Adjusting the valves is about a 15 - 20 minute operation.

3) Funny business with the cam gear. The gears wear out. The fiber outer sections become separated from the steel hub. The brass thrust washer weare giving it fore/aft slack. All these can lead to a fairly noisy diesel-like clatter that can sound quite serious. Could be mistaken for a rod knock, but it usually goes away at higher rpms as the oil pump puts the camshaft under a constant pull. At lower RPM's the valves can shoot the cam forward as the lobe passes TDC - the resulting back and forth torque puts any slack in play. Luckily cam gears can be replaced with the engine in the car (probably worth the while to pull the radiator for room, however). Cams (worn lobes are depressingly common on high mileage B18/20 engines) can be replaced in situ as well, but you need to pull the head to remove the lifters. You should always use new lifters with a new cam, unless you happen to enjoy swapping cams.

4) Bottom end problems. As serious as it gets - worn rod or main bearings will clunk with a very heavy hefty sound. Bearing knocks like this can usually be diagnosed from a cam gear noise by when they occur. Cam noises are worst at idle, and fade as the revs increase. Rod and main clunks can occur at higher rpms, and are worst when the engine is slowing down (on the overrun). Rev the engine up, then close the throttle quickly and it will rattle worst as the engine slow down. Also the pistons can become a little loose on their wrist pins and slap against the head. Full rebuilds are the only way to cure these sorts of maladies, although used car salesmen sometimes find gear oil works wonders too...