My 2007 Mazda BT-50 makes a slight pinging-like noise (not obstructively noisy and not heard from passenger seat) at light acceleration on the highway. When the throttle is depressed just a little more, the noise disappears. I use Liqui Moly diesel additive and this appears to make some difference, but the noise is still there. I have had the car since new, but only really noticed it in the last year or so. It has just done 120,000km and had the timing belt replaced. Can you shed some light on what the noise might be?
It's a diesel, so it's not pinging. I suspect it's the injectors rattling. Have a diesel mechanic check them for you.