Park Slope Mysteries: The Case of The President Street Vibration

The Mystery TeamHey you, gumshoe. We've got a new case that needs crackin', see? A tall, gorgeous woman woman breezed into our FiPS office, see? (OK, she wrote an email) -- anyway, this lady's been noticing a "strange vibration" in the Slope and wants to know if anyone else feels it too:

Okay, is it just me? I live on President street between 8th and PPW and have noticed this strange vibration like a distant subwoofer, night after night. I can't locate where it is coming from  --- does any one have any leads? Every night I can hear a low grade vibration that sounds like a car going by with a subwoofer and can't locate what's causing this sound. I can, in fact feel the vibration from this sound. But when I try to investigate where it's coming from, it's hard to locate the origin as it doesn't get louder when I walk outside or if  put my ear to the walls. No it's not a "flashback"! I hope I'm not alone, as I would love for someone to say: "it's such and such"...and solve this weird mystery. 

So?