Engine Noise in Fifth Gear on Highway

[kueifan]

Hi Folks,
I have a 2009 Automatic Mazda 5 Sport. I'm hearing a noise from the engine that sounds like a diesel truck, sort of like a quick splutter, "thud-thud-thud".

The first time I heard it, I was driving across the Whitestone bridge on I-95 and thought it was a nearby truck. The windows were up, so the sound is not very loud and seems like it was coming from outside. However, I've noticed the same noise while there were no trucks around and realized the noise was from my car.

The noise is only noticeable when I'm in 5th gear and the rpm is between 1,700 and 2000. Above 2k rpm, the noise either goes away or gets drowned out. The noise is slight, but noticeable if you're looking for it. I can be cruising at this speed and the noise is constantly there. I don't notice the noise while parked and rev'ing up the engine to the 1,800 to 2,000 rpm range.

So, my questions are:
1. Does anyone else experience/notice this? Is this common or unusual?
2. What could be causing this? It seems like there's an issue with the transmission, that the engine is getting overloaded while in 5th gear and it should be in 4th instead.

I had a 30k mile maintence on the car at a dealer recently and they didn't mention anything. I brought up the issue, but there was snow on the ground and they didn't want to take it out for a test drive on the highway to replicate the issue at that time. I plan on going back eventually to have them take another look, but would like to be informed before going in.

 

[thaxman]

If its what I think it is, then I am surprised its happening close to 2K. I have owned many 4-banger auto-tranny cars, and it seems that whenever I would drive under load (like uphill) in the top gear at low speed (like 30 mph) that I would get this knocking from the driveline, that sounded like a diesel idling. But usually that would be about 1500 rpms, maybe lower, IDK. Either way, don't drive below about 2K under load. Coasting downhill or level is fine, but use your manu-matic to keep it at least in the torque curve uphill.

 

[flcruising]

The symptom you describe is called knock, ping or predetonation. That's what I suspect. The one simple test you can do to determine if that's the case is to fill up your next tank of gas with 89 or higher octane fuel. If the noise is minimized or eliminated, then you either have excessive carbon deposits in the cylinder or your knock sensor could be faulty and the PCM is not compensating.

 

[thaxman]

Yeah, I'd go more with what he said. When you are in too high of a gear under load, its loud enough to be heard over talking or the radio.

 

[kueifan]

Thanks Guys!

I will try out the higher octane trick to see if the noise goes away and report back. I don't drive much, so it's going to take me a week or so before I need to fill up again.

Sorry I didn't reply sooner, I thought I would get e-mailed when people respond to my post, but I guess not. I'll have to check my forum settings.

 

[reMarkable]

Since my '08 Mz 5 Sport Automatic hit about 5k miles (it's now at 65k miles) - I've heard a knocking/pinging/a cricket-like buzzing under anything more than the slightest load - right at 1,500 rpm to 1,800 rpm - regardless of gear. I've asked about it a few times here on the forums and gotten little response. It does improve a bit with high octane, but doesn't go away. (Side note: I get about 26 mpg average, but I do a combination of about an hour's driving in NYC traffic and then another 1.5 hours worth of driving on the NJ Turnpike a day.) A lot of my city driving is right in that rpm range - so the sound makes me nuts - and lately it seems worse somehow. My initial reaction to the sound two and a half years ago was to switch from buying gas at the cheapest name brand station to sticking with a single brand that promised a cleaner engine - so based on my travel habits - I aim to fill up exclusively at BP. It hasn't cured anything - although I suppose it could have gotten (much) worse. I've also tried using Techron a couple of times in a row to theoretically help clean things - but it didn't really seem to make a difference. I haven't investigated the knock sensor - although maybe I should - even if it hasn't thrown a "Check Engine" light/code up. I'm just reluctant to have anything to do with the dealer unless absolutely necessary.

I'd love to hear if anyone on this topic - especially anyone who's successfully cured this in an Mz5.

 

[kueifan]

I filled up the tank with 93 octane (it was still 1/4 full with 87) and the noise seems to have gone away or at least improved significantly. Sounds like I have some thing to go on when going to the dealer. I'll let you know how it goes.

 

[Robotazky]

^^ FYI, BP is not a "Top Tier" gasoline. It may be OK, but it's not up to specifications that allow BP to call it Top Tier.

 

[reMarkable]

^^ FYI, BP is not a "Top Tier" gasoline. It may be OK, but it's not up to specifications that allow BP to call it Top Tier.

Very correct - notice I never used the words "Top Tier." (I'd consider it - but there only seems to be Shell in my area - and there aren't any vaugely on my route to work nor in my neighborhood.)

 

[kueifan]

Folks, I'm been living with this noise for a while now. I took it to the dealer and was unable to replicate the noise problem with the service manager. He gave me some suggestions (which included the higher octane suggestion) and sent me on my way.

Well, I tried the higher octane again and this time, the knocking was still there while in 5th gear at around 1,500 rpm (45mph), so I guess the issue might not be the knock sensor.

At some point I'm gonna try to figure out where around the dealer I replicate this sound for the service manager, but I just haven't gotten my lazy butt to it yet. It's difficult around NYC to get some stretch of road where you can maintain that speed without hitting a red light or another car. There's a lack of hills in the immediate area.

I'll report back here if I ever find a solution. Right now I'm just wishing that little engine light would go on and give a diagnostic reading.