No way to identify your problem without a more detailed description. This could be anything from a noisy lifter(s), other valvetrain noise, ticking injectors, rod bearing knock, piston slap, etc.
When is it rattling? Startup or all the time? Is it worse when you first start the car or when it warms up?
Is it worst at idle, or when you rev the engine? It it a ticking or a hammering sound you hear?
Even if you have low oil pressure from worn bearings, you can buy some time with much heavier oil (20W50 or higher in the Texas heat). That's not a permanent solution, but I wouldn't accept the mechanic's advice without a 2nd opinion (I'm sure they'd be very happy to install a rebuilt engine). You could also pull one of your valve covers and see if there's any truth to the oiling guess.