That sound is just Dapper Dan and Diamond Lil, the parents of three baby peregrine falcons that nest on the ledge of the 33rd floor of the building at 77 Beale St. Loud noises are their way of trying to ward off University of California, Santa Cruz biologist Glenn Stewart as he attaches bands to the baby birds' legs.
"I'm trying to learn three things," Stewart said. "How long they live, nest site tenacity, and -- probably the most interesting thing -- dispersal, or how far they go from their nest."
Dapper Dan and Diamond Lil have been residing at the Beale Street building for four years, while other peregrine falcons have been nesting there since 2004, and biologists are still not sure why, Stewart said.
What they do know is that this particular nesting couple and their three babies, or eyases, who are 21 to 23 days old, are extremely popular with the public.
"They're cute, white, little fluffballs right now," PG&E spokesman Joe Molica said.
Stewart estimates that about half a million viewers tune in each week to watch the birds via a live camera on the university's website, making it the school's most regularly watched site.
Peregrine falcons are predatory birds that eat other birds caught in midair. They can dive at speeds of up to 200 mph, Stewart said.
The number of falcons in California had dwindled to only two nesting pairs in 1970. Now, thanks in large part to programs in which biologists bred the birds in captivity and released them, researchers estimate that there are about 250 nesting pairs in the state.
Dapper Dan and Diamond Lil's brood of young falcons -- who will be named once their sexes are determined -- will get a band on each leg. One is for the U.S. Fish and Wildlife Service, with a phone number to call in case one is found. The other is a visual identification band that can be seen from a distance through binoculars -- this band will help watchers determine how far the birds roam.
Interested bird watchers can view the banding process live at 11 a.m. on the "falcon cam" at http://www.scpbrg.org/.
Also Thursday morning, UC Santa Cruz biologists will attach ID bands on the legs of baby peregrine falcons in San Jose.
The four babies live on top of San Jose City Hall. The last hatched about three weeks ago.
After the banding, the team will determine if the young falcons are males or females.
Scientists say the bands do not hurt the birds. They are used to monitor their whereabouts and keep track of the population.
There are an estimated 250 peregrine falcon nesting pairs in California. There were just two known nesting pairs in the state 40 years ago.
ABC7 contributed to this report