It does it on my Powerbook as well. It isn't a software issue, it is a hardware one that just happens to become evident when clicking a link in OmniWeb. Whenever the CPU is maxed at a certain threshold, certain components emit a high pitch (a similar effect to the MBP "whine"). If you want to have fun with this, try the freeware
SystemLoad... if your Mac suffers from this problem, you will hear a scale play (not from the speakers, from the actual components) as the CPU ramps up.