1988 Ford E-Series Van feel heat and smell unknown odor top

Notice heat and a smell, not like burning oil or rubber,
coming from right upper area of engine, noticeable thru the grill.
Checked: radiator full, fluid green. Overflow just a tad low. Refilled this. Heat gauge not higher than usual.
Oil a bit dirty altho changed less than 500 miles ago, last year. Under van, wiped with pper towels, can't find any drips of oil, tranny, or any fluid.

Serpentine belt seems taut, no nicks or fraying. Was replaced in 2004. Van driven only 2K mi since then.
Had complete tune up last year, new sparks, plugs, rotor. Throttle was checked.

New fuel pumps and filters last year.
New calipers, flushed brake system, serviced differential Sept 2009. Checked rear end fluid and wheel bearing repack then. New tranny fluid.

what else could cause heat/odor?

One mechanic we don't trust (overcharges) Sept 2009 said we need Valve cover gasket set, wanted
$664. We declined this service, have not seen smoke
or noticed any unusual heat till now, Aug 2010.

appreciate any suggestions for possible sources of heat/odor!