91 Honda Civic intermittant whistling noise

Hi, I have a 1991 Honda Civic (5-speed) and it is a great little car. Despite its mileage we have been told by our last mechanic that he expects us to get another 60-80,000 miles out of the car.

Recently it would not start one cold morning and so a mechanic came to check it out and ended up installing new spark plugs, which fixed the problem. The next day I was driving it when suddenly and out of nowhere a REALLY loud, high pitched whistling noise started coming from the engine. It lasted about 10 seconds and then shut off as suddenly as it had started.

Its been about 3 days since that first time and since then it has happened at least once during every drive I have taken. Sometimes for only 10 seconds, sometimes for an unbroken 5 minutes at a time. The whistling occurs regardless of what gear I'm in, even when the car is in neutral or at a standstill. It seems to come and go randomly. Its new years weekend so NO car places are open here, but I want to know how serious of an issue this is! Should I still be driving it around?

Thank you so much in advance!