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

Car is making a noise Inspection $79.99 - $89.99 Get a Quote
AC is not working Inspection $96.46 - $106.46 Get a Quote

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.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

What others are asking

Hi. Jose I got a 2007 bmw 530i and in opinion for year of the car is maintenance very expensive for those cars?

Hi James. Thanks for contacting us today. As the owner of a 2009 BMW 128i that just had it's last payment made, I can tell you that routine maintenance and service for these vehicles is on the average, 30 to...

on/off humming sound over 65mph

A "humming noise" while driving at speed can be caused by tires or wheel bearings. If you drive with the window open and you hear the noise outside the car, like the sound bouncing back at you from a wall,...

My truck cuts out and jerks when lightly applying accelerator at 45 to 60 mph. If I gas on it hard it doesn't do it

It sounds like you may have either a failing throttle position sensor or potentially a dirty or faulty mass air flow sensor, both of which can cause a bit of a jerking reaction when not functioning properly. The mass air...

Volvo s40key was stuck won't crank clicking from fuse box

It may be that you have low system voltage. That might be because of a dead battery, or it might be because of an interruption in the power distribution. But since everything is computer controlled these days, if the system...

I have a 3.6L leo commodore. I'm getting an error code of p0040. I need to swap the sensors plugs. Is it easy to find the connections?

Hi there. The code p0040 is for an O2 sensor connectors are being swapped. Make sure the O2 sensor connectors aren’t swapped. This can happen when work has been previously performed. Also, check for unplugged or damaged connectors, as well...

Car doesn't always start right away

Hello. There could be a few issues causing a crank no start condition. You could have a sensor that is failing when it gets hot like the crank or camshaft sensor. A failing ECT sensor can also cause a crank...

Noise in the front end while turning the steering wheel

This is a common sound when the ball joints begin to wear out. As the ball joints are subjected to various loads and pressure depending on the weight of the vehicle and force/torque of each turn, this squeaking may vary...

Electrical system malfunctioning

Hi, is there any more information you can give us? These types of cars tend to have a multitude of electrical systems in them so there are many things you could be referring to. You could try resetting the computer...

hearing high pitched loud whistling (NOT squealing) sound while driving up hilly neighborhood

Hi there. Generally speaking, the mechanic you contacted is correct; that's it is very hard to diagnose a sound or noise without being able to replicate the source of the noise. This is why we offer mobile mechanic services, so...

Related articles

Does Running My AC in the Summer Keep My Engine Cooler?
When When you engage your air conditioning in your car, it is usually summertime and hot outside. Air conditioning serves to remove heat from the inside of your vehicle by circulating a refrigerant through the evaporator core in your dashboard,...
Why Won't My AC Blow Cold Air?
When you turn on the air conditioner in your car, you expect to receive a flow of fresh, cold air....
How Long Does an AC Compressor Belt Last?
For For the air conditioning system in your car to work properly, it has to pressurize refrigerant. That is the function of the AC compressor. The compressor itself gets a signal from the AC relay, and is operated by a...