I have searched this subject and it has confused me even more.

The situation:
When driving down the road (hills or no hills does not change it) under 3000 rpm when i press the gas I hear the rpm's rise then fall back off, it slightly sound like a tranmission slip however I do not seem to loose power. On top of that around 35-42 my normal driving range it "sounds" like it is going in and out of overdrive. I noticed it after I changed to a magnaflow muffler a few months back. Does anyone have a idea of what could be going on?

My Car:
2008 LT CAI, Maganaflow Muffler, 58,000 miles, 17x7 Sport Edition A7 Wheels w/Fuzion HRi Tires, and Auto tranny

 

Most likely you're hearing the torque converter locking and unlocking combined with a back and forth shift between 3rd and 4th, the louder Magnaflow exhaust amplifies the engine note enough to make it noticeable. The shift from 3rd to 4th can occur at 38 mph with light throttle application, 12.5% of the throttle travel to be exact, the torque converter can lock up at 25-27 mph.

So its entirely possible the you've found the particular speed range at which your transmission will "hunt" between 3rd and 4th gear while the converter is also locking and unlocking in response to the throttle position and engine rpm.

Unless the transmission begins to slur the shifts between gears, or starts to really slip(you'd know it if you ever felt it), I'd not worry about it. Of course for absolute peace of mind you can have it hooked up to a scan tool at an auto parts store.

 

Seems similar to what I have noticed with mine after I added the airaid intake. Im starting to think this is normal, and just more obvious because the engine noise is more noticable.

 

I don't notice it on the '07 or the '11 with the GMPP exhausts and intakes, but on the '06 with the Magnaflow exhaust you can get a resonant booming at certain rpm ranges which coincide with the shift into 4th.

 

I was thinking along thoes lines also. It isnt having problems shifting as of now maybe I will hit a part store soon just to be safe. and thank you :)

 

I am also hearing a loud humming/whining from my car which I thought sounded like it wasn't shifting properly. Two months ago I took it to the dealer and told them about it and they said it was bad tires. Now I just replaced the tires and the sound is just as loud. It starts out as a low humming sound then becomes a higher pitched whining sound at higher speeds. It sounds louder on the drivers side. I have 64,000 miles on my HHR. Anyone else experience this?

 

Does the noise change when turning/swerving from left to right?

 

You mean like Autozone, etc.? They're not going to have anything that's of any help at the auto parts stores unless you're showing a CEL and need to read the code. You'd have to go to a GM dealer or somewhere where they have a GM TECH 2 for that type of diagnosis.

 

Mine does that kind of thing too, it's just trying to be in the right gear at the right time. It must be more noticeable with less power. I can usually pick the right gear to stay in, with the gas peddle. Some times all the technology needs a little operator assist. I really don't notice the trany doing it's thing in my v8 car, I'm still impressed what they did with a 2.0 4 cylinder.

 

In all my GM cars I can feel the Converter lock up and unlock. In the HHR it is a little stronger in it. When I go down hill coasting it feels like it shifts but it is only the lock up. If I had a loud exhaust it would for sure change the sound or tone of the engine.

You often can get the same results by keeping on the gas and just lightly stepping on the brake as it unlocks the converter.