2002 Nissan Frontier Engine is louder/hissing-whirring nois

I unfortunately had the circumstance present itself where I had to drive through a creek (sbout 2-3 feet deep) for a few moments, and then drive about an hour at highwayspeeds home. The next time I started the car, or at least when I first noticed it, the engine sounded louder, a sort of hissing/whirring in the backgroud, but does not seem transmission related (happens at all rpms, clutch in or out.) No loss of performance. Upon inspection, I did see I was missing the light plastic guard under the engine, but saw no indications of damage. I changed the oil, but it made no difference. It's not like a thumping noise, but a constant sound, sort of like when you turn the wheel on an older car while its not moving, and hear the whirring hiss of the power steering. Help?