Nissan Repair: Tapping sound coming from engine, 2005 nissan altima, viscosity oil

Question
QUESTION: Hello
I Recently purchased a 2005 Nissan Altima 2.5s about 5 days ago,with 143000 km on the engine. I noticed today  that a tapping/knocking sound is coming from the engine. When I left the car running and tried to listen for the sound under the hood I can't hear it, but when i sit in the car I can. I bought the car from the original owner and he has all the service records, the crank sensor, and the timing sensor was recently replaced. What do you think is causing the problem?

ANSWER: Shiva,

I would look into getting the oil inspected.  If you have a low viscosity oil the engine will make noise.  5W30 is the recommended oil for your car.  I would change to Mobil 1 5W30 Synthetic and you should be fine.  If not email me back and we'll discuss further.  I run Mobil 1 in everything I own and that includes my lawnmower...

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QUESTION: Hello

I changed the oil to mobil 1 as you suggested and the engine works fine now, thank you so much. I have another problem now, the car is idling rough when still in drive and i come to a stop i notice the RPM's goes up and down. What do you think is causing that, again your help will be greatly appreciated.  

Answer
Shiva,

The car has variable valve timing and the oil change could correct this with some time.  Otherwise you could look into a tuneup as in replacement of the sparkplugs, air filter, and Positive Crankcase Vent Valve.  Basic tuneup will aid in the correction of the rough idle problem.