Help needed: Booming Resonating Noise in Cabin at idle and low rpms

Good Morning all- I'm at my wit's end. I own a 2007 Prius Touring w/120kish miles. We are currently experiencing a huge booming resonance at idle and low RPMs. The noise isn't subtle, almost sounds like we're in a Huey with the doors open. Here's what I believe I know:

- No DTC's are bing thrown
- ICE seems as smooth as ever
- No apparent changes in fuel economy
- Plugs were replaced 40k ago and coil ignitors are orginal--not runing rough or missing though
- Visual inspection of exhaust shows no leaks or contact with body
- Motor mounts pass the test I looked up [in drive/reverse, depress brake, watch unuual engine displacement]
- Noise is not vehicle speed related, only engine speed coupled--probably between 1K and 1.8k rpm

I'm sure I'm leaving something out but aside from messing withteh coils or plugs I've loked at everthing I can think of. I haven;t gone to the dealer yet. Lot's of reading searching have only revealed CV issues. Any good ideas out there?


Cheers,
Brian

 

Good idea--done that before. No need to rev, the noise is present at idle, however, we have forced the noise to grow by putting the car in gear and reving the engine with the hood open. The noise isn't heard outside of the car at all. The engine seems as smooth as it has ever been. A/C on or off--no change.

Keep the ideas coming....

 

Yes, the AC works like a champ.

 

OK....all great questions.

- Exhaust was tested while the car was running. One person with hand over tailpipe and me under the car listening for exhaust leaks. I thought teh donut {i the prius has one} between the header and exhaust was bad. No apparent leaks anywhere.

- "Maybe the exhaust is rusting out internally"---maybe but there's no holes.

- So we live in New Hampshire, USA on a 1/2 mile dirt road--there's snow (not recently), potholes, rain, puddles but no flooded parts or deep water.

- The window reverb effect. All windows closed, or even open--it still happen. I own a 5-sp TDI that it's quiet easy to slightly open the rear windows and get a wonderful cabin resonance going. I'm familiar but this isn't the case.

-BTW, when this happened. It happened within 1 mile of the house on an asphalt road and just started out of the blue. No pops, grinds or dips in the road. There wasn't noise and now there is.

I changed plugs, if only to pull the coil packs and reseat them and there's no difference. Next I'm going to do a visual inspection on the harmonic balancer. I know what a good 350 Chevy balancer looks like but not a prius one....


More to follow...

 

- It is an ICE only noise---stone quiet when ICE not running
- Noise only at certain ICE RPM's-- idle to 1800ish rpms
- I did inspect the dogbone/wishbone mount and it appears ok, likewise for the passenger side mount, couldn't really see the mount under teh inverter to tell anything
- Exhaust-- no apparent leaks, hangars are all intact, and not rubbing in the body anywhere

Just cleaned the MAF and look at the engine drive pulley---couldn't really call it a balancer


I'm close to throwing in the towel.

 

I'm still leaning toward an engine mount or exhaust simply because the engine runs fine and is smooth as can be with a hand on it yet at the same time in the car it will will be booming.

How do I diagnose a broken or failed mount? What other symptoms are there? I've only seen 1 thread and I followed another posters troubleshooting method as I noted in my 1st post [open hood, car in Drive and/or reverse, use the accelerator holding the brake, listen for crunching/popping].

What else can I look for on the exhaust? Could I have a plugged Cat? The rear most part of the exhaust is new in the last 8 months--essentially cat-back.

 

It's cyclically in that it only occues when the ICE is running between idle and 1800ish rpm. It is not random, always occurs in that area of the power band. Lemme see if I can capture it.