I have a 2001 Nissan Altima it makes a short (less that a second) hissing noise ever so ofter and when it does the car kind of hesitates or hiccups ever so slightly. Any ideas of the problem?

Check all vacuum hoses for leaks.

What ever it is using for an idle air control device (at idle) maybe causing this sucking sound when it closes too much making the engine lean. Do you experience this problem while driving. A vacuum leak should not effect very much while driving unless its big but this does not sound like your problem, as mentioned a leak during idle would. More info on when this happens, cold engine, warmed up, what.

It happens when driving at 50 to 55 mph when the engine is warm and not at any specific frequency.

Can you guesstimate where the sound may be coming from, inside the car, under the hood, it would be strange for this to be a vacuum sound at random, could it possibly be an electrical arcing sound?

Random hissing from a vacuum leak may be the purge hose on the evaperative canister, when the evap solenoid opens semi random, just a thought. Also at cruising speeds is when the EGR operates semi random also, it has a vacuum hose, sorry I am not familiar with the exact setup on your car specifically.

Funny you should mention the EGR valve. My check engine light is also on. One technician told me the EGR valve was okay, but and I'm not sure I'm telling this right, but the bottom hose was probably closed. I had that checked or fixed (supposingly), the light went off for a week or two, still had the hissing sound occasionally, and now the check engine light is back on.

Get that code pulled for free at Autozone and let us know what it is please and lets go from there, it may fix your light and the hissing sound, lucky you.

My 2001 Nissan altima is doing this exactly too :(