04 I35: Weird engine noises - advice needed!

[Grand Moff Joseph]

OK, I'm at wits end trying to track down the random noises that this car has been making over the last couple of months, but the last two days have taken the cake:

Yesterday morningIt's very cold outside, around 15-20 degrees. I start the car, expecting to hear the usual elevated tick from the engine, as it does on cold mornings. However, I hear what sounds initially like a horrible rod knock.

This knock settles down to more of a slap within 20 seconds, but there is a constant moderately loud slap sound going on. In addition, the exhaust smells like crap. I towed the car to the dealership. they reproduced the issue, but once the car warmed up, it went away totally. No codes thrown.

Last nightGo to start the car (again cold start), and I get two new noises:

1. Low volume slap while engine is idling, smooths out some when warm, but still there2. Before I started the car, I turned the key to ACC On. Instead of the usual fuel pump priming sound, I heard three fast squeak sounds, pretty high pitched. Almost sounded like a high pitched "wrong answer" buzzer3. When idling, if I goose the gas, then let off of it, I hear the same three squeaks as the engine revs drop.

Dropped the car off at dealership, and the service techs heard the noise. they started it this morning, and they now say the noise is gone, and they see no issues. I'm desperate for some help. any ideas?

NOTE:All fluids are fresh, air filter is cleanNo service lights on at all

[MaximA32]

What weight oil are you running?

[F8NBTHR]

I'm hearing a backfire sound coming from the rear. Someone told me it was air in the gas tank. Have you heard any fix for this.

Andy

[Sentientbydesign]

Air in the gas tank???!!!

Of course there's air in the gas tank. It's not a vacuum sealed system

Maybe they meant air in your fuel lines.

Doesn't a backfire usually indicate uncombusted fuel?

[F8NBTHR]

Thank you for the reply. Funny thing is, I can not duplicate the sound when I want to. It just happens at different times. I have noticed it when the tank is full of gas, it does it less. It sounds like it is coming from the tail pipe. Any thoughts on what to do or tell the service center?

Andhy

[Grand Moff Joseph]

What weight oil are you running?

Valvoline SynPower, 5W-30.

Sorry for the delayed response...have been sick lately.

Anyway, after doing more research on this site, Maxima.org, and a few other Nissan sites, it looks as though the electric noise I am hearing is the electric motor mount.

Any ideas on replacing motor mounts regarding cost?

[MaximA32]

Ah, makes sense now. The electronic motor mounts on those have always been noisy. They're not easy to replace but I have no idea on the coast as I've never had to buy them personally.

[Sentientbydesign]

Thank you for the reply. Funny thing is, I can not duplicate the sound when I want to. It just happens at different times. I have noticed it when the tank is full of gas, it does it less. It sounds like it is coming from the tail pipe. Any thoughts on what to do or tell the service center?

Andhy

Let's stop thread jacking and start a new thread with your issue. You'll probably get a better response.

I'm really at a loss of ideas on what your issue is though.

You could have them do a full test on your fuel system, but that could be costly.