Drew Barrymore -- Lights, Sirens, Action Over Suspicious Van At House

Drew Barrymore had a big scare Tuesday night ... a white van pulled up to her L.A. house while she was home alone with her child and the person inside seemed intent on getting her to answer the door ... repeatedly knocking ... and it triggered a panicked 911 call.

It sounded ominous ... but turns out it was one of your friendly AT&T service guys working in the neighborhood. The pregnant Drew didn't call for service and it's unclear why the dude was knocking, but after cops came and helicopters hovered, they found the van several blocks away, checked the guy out and all was ok.