Sitting outside or getting rained on didn't likely have anything to do with it, with one exception. Rain could have shorted out the ignition system, and one or more cylinders might not be firing right. Goldwing engines sound that way when all cylinders are not firing, at least the four cylinder ones. Check for a spark at all four plugs first. Also if you do not know their condition, check the belts. In 30 years, even with such low mileage it is possible for them to be badly deteriorated, also the tensioners could be stuck. Hopefully the belts did not slip. If the belt cover seal is bad, water could have gotten in there as well, and a wet worn out belt is more likely to slip than a dry one.