Q: Pressed the gas pedal and the car didn't move forward. It rather rolled backwards because the vehicle was on a hilly place. What cold have happened?

I was at a stop sign ready to Turnot left. I put on the signal to turn left. It gave me a continuous pop pop loud sound. The same with right. Then when I took my foot off the brake to the gas pedal the car started rolling backwards. This happened once . But it's been driving well after that. INot addition in 3 occasions I press the gas pedal and brakes it gave me a warning sound .but moves on though. And the radio shuts off for a second. (The warning sound is You know that sound the car gives when u leave the door open with your key in ignition hole)And then finally the car just won't crank. So called a mechanic who checked and saw that the was corrosion in the line from the battery to the car. He cleaned it and the car started. Well my main concern is the gas pedal. I don't think it's loose. What could have been the cause of it not picking up once. And the odd sound from the signal. Could all these be related to the battery cable

My car has 157560 miles.
My car has an automatic transmission.