Loud COW-MOOING SOUND AND Check Engine Light

Hi, I just bought a 2000 MPV in Florida, and it is now in New York. Last night, I drove it out, and only 2 minutes later, it started making weird and loud cow-mooing sound (that seems to be coming out from the engine room) for about 5 minutes. I turned the ignition off, and restarted the engine, and that weird sound went away. Then, I drove about 2-3 miles, and the check engine light suddenly went off. I wonder why I had CEL on. Is a reason for CEL related to that weird and loud cow-mooing sound? I am concerned.

Thank you very much for your help in advance. Thank you, thank you, and thank you!

The noise ( loud cow mooing sound, excelent description by the way) is from the Idle Control Valve. Very common problem sound and a replacement valve will not make this noise. Will not cause any problems to you but is just an anoyance. Should not have turned on the CEL for this problem as have never seen this happen but you never know. If the van is driving the same as it was before then the CEL is just pointing to a problem with emmission system so should not be a concern. Get the code number checked and we will see what it means.

Thank you very much for your quick help. I'll go to a local Auto-Zone to get CEL codes, and will find out what the causes are. Hope it is nothing or very minor issue. 

Get back here with the code numbers and we will see what is going on with the MPV. Do not be alarmed by the CEL. Could be something as small as a fuel filler cap not being fully tightened by the gas jockey last time you filled the fuel tank.

I checked the CEL code, and it was P0136 causing the CEL turned on. Here is the description I got:
"The PCM has determined that a malfunction exists in the electrical circuit for oxygen sensor 2 in bank 1."
Defintion: H02S 12 circuit condition (heated oxygen sensor bank 1 sensor 2)
Explanation: The powertrain control module uses the oxygen sensor to calculate the air/fuel ratio in the exhaust.
Probable Causes:
1. Oxygen sensor defective
2. Fuel system running very rich or lean-check other codes first
3. Engine misfire condition-repair
4. Fuel pressure very high or low-fuel pump or pressure regulator

I have a few other things to report on this car.
a) the exhaust which is right next to the motor is creating a noise (fast and continuous, as well as relatively loud, tapping noise) when hit the gas (not when idle), checked by a mechanic, was told it was nothing major, no need to worry.
b) I had ignition problems several times, had the problem last night and this morning too. When turn the ignition on, I could hear grinding noise, and the car won't start. I tried it again, it still didn't start. Then, I set the key on ACC for few seconds, and turn the ignition, then it starts. It starts fine thereafter. Last night and this morning, it was a little chilly in Buffalo, NY by the way. This morning, when I started the engine, the temperature needle was way below "C", and the rpm went up to 2,000 rpm. I let the engine run idel for couple minutes till the rpm dropped down to 1,000rpm and the temperature needles went up above "C". Changing gear (by hand) seems to be less smoother when the vehicle is not warmed up.
c) I uses 89 gas in general, and used 93 gas last time when I filled the tank along with a small bottle of fuel injection cleaner that I got from CarQuest. Can using too high octaned GAS cause the CEL?
d) This morning, I had battery and alternator checked at Advanced Auto Parts, and they were both fine.
e) Last week, two dash board bulbs were blown out, and they were replaced. Can this be related to the CEL? Also, my triptometer/odometer stays on for a while (at least several minutes) even after the engine is shut off. Is this normal for Mazda MPV? I feel it weird though.
f) One last thing, I turned the heater on last night because it was quite chilly. Coincidantly, the CEL went off last night.

I asked an AutoZone guy to reset or clear a code, but he couldn't do it. Do I have to replace the O2 sensor right away? Should I wait for several days or couple weeks to see if the CEL automatically goes away? What would be the average cost for removing the CEL in this case (replacing O2 sensor)? I hope it won't cost too much.
I am sorry that I have so many questions. I thank you and I appreciate very much.

The P0136 code is actually indicating a malfunction if the signal to your "middle" O2 sensor. Your van has 4. One in each exhaust manifold, One down under the car in front of the cat converter and the last one is located just after the catalytic converter. The "middle" sensor is the one down under the van in front of the cat converter.
A faulty sensor, vaccum leaks, or excessive fuel could all potentially cause this code to set.
However, you also mentioned that you hear a "relatively loud, tapping noise when the engine is revved". This sounds like an exhaust  leak from one of the exhaust manifold gaskets. Funny thing about exhaust leaks is that they not only allow exhaust out but also allow air into the exhaust system. This air that is being drawn in could be diluting out the exhaust with oxygen and creating the exact set of conditions needed to get that middle O2 sensor to be inactive which would then in turn set a code P0136. Think I would look into getting that noisey exhaust leak fixed and would be willing to bet that the code does not reset. 

Thank you, babyhuey!
 
There was a leak in the right exhaust manifold, and the mechanic welded it. The CEL code was cleared. I drove the car for about 70 miles, and the CEL has not come back yet. Hopefully, it won't come back again. Once again, thank you very much, babyhuey!

Glad my exhaust sounds fine. But it had been clogged before due to its damaged Mazda down pipe. Since the pipe wasn't functioning well, there was a backflow that caused the particles to clog some components. It also produced some weird sound but it went away the time I replaced the downpipe.