Depending on the depth of the tunnel and the construction of the structure, you can get vibrations through the ground and foundations of the structure transmitted such that they are noticeable.
e.g. If you live above a tube line in London (London Underground) then you may hear/feel rumbling every time a train passes under you.
Making the route from San Diego to LA high-speed is perfectly doable (the route exists already), and would be a great stepping stone.
Even if it turns out the Tehachapi is basically the mountains of Mordor and the project ends, you'd still have a valuable high-speed corridor.