Is it possible that you have an exhaust manifold leak? Why don't you
look at where the exhaust manifold bolts to the engine block an see if
you see a spot with black carbon buildup. If so the gasket between the
engine block and the exhaust manifold needs replacing. If that looks
o.k. I'd start inpecting the manifold itself for cracks.

BTW, an exhaust manifold leak will typically not generate any problem
codes unless it is really bad - i.e. bad enough to cause a significant
drop in exhaust pressure.

B.B. wrote:

> I'm not sure how else to describe it or how long I've had it since my
> radio only recently died. (: It wasn't there before, but now it makes
> a distinct huffing sound--kind of like an air compressor does, though
> quieter. Not very loud and goes away (or is covered by other noises)
> when driving at speed. Engine temperature doesn't seem to affect it.
> Should I care? And if so, where to start looking?
> '98 Honda Accord--no codes from the computer and just passed an
> emissions test a little while back.
>