Q: Strange rattling/tapping noise at low rpm when accelerating lightly

asked by on April 15, 2017

I hear a strange noise when lightly pressing gas in park or in gear. Sounds like it's coming from the lower part of engine somewhere. My a/c has recently gone out also and blows hot. I was thinking maybe it's the compressor making noise. It goes away once moving at a higher speed/rpm. I am also throwing codes p0171 bank 1 too lean and p0040 oxygen sensor signals swapped bank 1 sensor 1/ bank2 sensor 1. I recently had the valve cover gasket changed also as it was leaking alot of oil for a while apparently since previous owners. the car's power seems to be very strong and performance doesn't seem to be lacking at all though. even with the codes it's still very fast. what could this strange rattling/tapping sound be? and what could be the cause of the a/c not working? I tried to recharge a/c low port but still blows hot.

My car has 73150 miles.
My car has an automatic transmission. 2007 335i sedan

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Hi...the noise can be diagnosed by taking all the belt driven accessories out of the picture by momentarily running the engine without the serpentine belt. If the noise remains, then the search is focused on internal engine parts. If the noise disappears, the mechanic inspects the belt driven accessories. Often noises are in the accessories driven by the belt and that’s why that simple, initial strategy is suggested. If you request a noise diagnostic, the source of the noise might be really obvious to the mechanic and so it might not even be necessary to remove the belt.

A/C systems are fairly straightforward. You might have a leak (all mobile, as opposed to home, a/c systems "seep" but obviously the "seep" rate is very, very small). If a leak is diagnosed, the leaking component has to be repaired, otherwise re-charging is a waste of your money and time. If the system is presently charged (the mechanic will determine the state of charge) and yet there is no cooling, typically that means the compressor is not working, a switch is faulty, there is a blockage, air/water in the system and so forth. To resolve the A/C problem, please simply request an A/C system diagnostic. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.

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