Rattling noise when gas pedal is pressed/driving

2009 Corolla S: had my oil changed a few days before a road trip and when I got back home(4 days later), noticed my engine making a metal rattling noise when I would press the gas pedal and it would remain constant at freeway speed. I also noticed when I'm at a stop light, the idle would occasionally be rough, feeling like it could stall any moment, but hasn't yet and no engine warning lights have come on. I've tried a few bottles of Chevron's fuel system cleaner and have bought the last few tankfuls of super unleaded gas to help clean the fuel lines in case it was from super cheap gas I had recently bought prior to road trip. Still makes the rattling noise. The noise seems to come and go, but definitely constant at high freeway speed. Toyota dealership wasn't sure of the issue, and said they would charge $1500.00 just to take apart the engine to try and find the problem. Help! :(