I have a obd1 B18C1 in my civic. there is a slight ticking/knockign noise coming out of it. No check engine lights are on. its not as bad when i am driving it, and it is in gear, but at a stop and i rev it i can hear it. it is also worse if i only tap the peddal and dont really hit it hard. any ideas? is it probably just the valve lash that needs to be adjusted? and how bad is it to keep driving the car like this?