I just got my 2003 MCS running from a bad oil cooler a week ago, and it rewards me with strange noises and a CEL. Possibly unrelated...

First day I drove it to work it started making a strange exhaust noise when I started it up to drive home. Louder and an uneven or "lumpy" sound. That morning and during a drive to get lunch it was totally fine. Coming home, after being on the freeway for 10 min the CEL comes on throwing P0420 (cat efficiency below threshold or something). The cat and both 02 sensors were replaced 5k miles ago....exhaust leak?

I let it sit for a day and then when I went to start it for a trip to the store, it immediately starts making a new noise that I can only describe as tapping or ticking. Its audible inside the cabin at idle and up past 3k rpm (I'm hesitant to take it much past that), and the tempo changes with engine speed. Outside the car, it sounds like a diesel. After few minutes to warm up, and a short trip to the store (5 min each way) its still making the racket. I have a video where its making the noise, but it does seem quieter than the first time I heard it, so it may be somewhat intermittent.


My first thought was rockers/lifters (I had an old Isuzu Trooper which are notorious for lifter tick when they're cold, so I thought I recognized the noise), but I've also heard of the timing chain making similar noises if the tensioner is going/gone. However, I dont think its the chain because the sound seems to come from the middle of the valve cover, not the passenger side.

Does anyone have any thoughts or advice before I dig into the top side of the engine? Or is it normal and I'm just crazy? Any chance the CEL code and strange exhaust sound is related? Thanks!

 

After listening to the video it sounds more like an exhaust leak, To pin point the source you can use an automotive stethoscope, about $10 at an auto parts store. But yes an exhaust leak will bring in a P0420 CEL. That is coming from the downstream O2 Sensor (after the cat) indicating that the cat is not working efficiently.

Here is good link explaining O2 function https://www.aa1car.com/library/o2sensor.htm

I had that CEL once and it turned out to be a defective downstream OS sensor.

 

Yeah I have seen a couple posts with similar symptoms that started as a question about an exhaust leak. I saw one where they described it as "suddenly my mini sounded like a WRX" which tracks. The header and cat were replaced by the PO about a year ago, but the cat back is old...ultrik brand or somesuch? They dont make it any more. I might take it to a muffler shop and see if they can find a leak.

I also got a video with the ticking sound more audible. Now it seems like its coming from the passenger side? idk what to think. Just hoping its not the timing chain...

 

my mini sounds exactly the same. been lkke that every since i got it 5 month ago. i think it's just general noise from valves tick or sticking or maybe that why you have p0402 because exhaust air not making it down right to downpipe and tripping o2 sensors, if you replaced cat and o2 , you shouldnt be getting P0402 catalytic below efficincy . how i know? because i had that code too. before i swapped my catalytic converter for a new oem on from magnaflow. my old exhaust manifold didnt have a cat at all. The guy i bought from cut it out. if it werer the timing chain then i'd think youd get a cylinder misfire code?

 

So the muffler shop found a leak in a gasket just upstream of the cat. I have no idea what kind of header/cat is installed...I know the OEM ones dont have a gasket there...anyway. Seems it fixed the issue I was having. No more codes, and the engine sounds normal again.

 

Check the bushings on the belt tensioner damper (black hydraulic rod). They dry out, crank then fall out causing it to rattle against the attaching bolts at both ends. Access is easy thru the passenger side wheel well. $16 fix. You do not need to replace the entire belt tensioner assembly if this is the problem.