Hello everyone,

I have a '99 996 6-speed with the loud noise mentioned in the title. It's in time with rpm and sounds something like the deep exhaust noise of a motorcycle or a very loud knock. It happens only when you're really heavy on the pedal; you can be accelerating conservatively with no noise, then depress the accelerator at least half way down and it will begin. It can also occur when the car is stopped with no gears engaged, but again only if you accelerate aggressively. When sitting in the driver's seat, it sounds like it's coming from the right side of the engine.

Car history: I purchased it with a rough idle which turned out to be a bad variocam on bank 1-3 that someone had messed with and replaced with a broken actuator and incidentally with the intake/exhaust chain set incorrectly. I had the variocam replaced, but I later found out that the shop put in the bank 4-6 part (causing the camshaft spindle to wear into the variocam housing) and didn't reset the chain but left it in the incorrect position.

About the same time, my water pump died and the bank 4-6 cylinder head cracked. Replaced with a junkyard part. The car never ran right and that is when the aforemention sound started. I tried to put the car in time and was unsuccessful. I also noticed that the mechanic didn't have the scavenger pump on correctly for bank 4-6.

Next, I broke down and took it to the dealer for evaluation. Turns out that the last mechanic also put the exhaust camshaft on bank 4-6 in place 180 degrees off. It was started on several occasions and I was mortified when I found out the cause the engine roughness. A full leakdown test at the dealer demonstrated no leaks (i.e. probably no valve damage), but compression was a little lower on 4-6 than on 1-3 which may be expected when using a junk yard cylinder head. They could not keep the engine in time. After setting one side, then the other with the appropriate technique, they started the car and heard a clunk and then found bank 1-3 to be out of time. They said they didn't know what was wrong but thought I should replace the engine. I diagnosed the clunk resulting in timing going out as a bad IMS and decided to get this fixed before scrapping the engine.

So... I found a fabulous mechanic (with an engineering degree) who has worked on several racing teams. He replaced the IMS which was definitely going bad and preventing the car from staying in time, replaced the failed air oil separator, fixed multiple vacuum leaks, fixed an electric drainage problem, fixed the bank 1-3 camshaft chain misalignment, fixed an exhaust leak on the bank 1-3, etc. The car now stays in time and has a smooth idle and strong acceleration, but I am still left with loud noise under load.

I'm hoping this is just an exhaust header leak. Any other thoughts? Hopefully not ones that doom my engine? Doubt crankshaft bearings because it lateralizes to right. Hoping no valve problem because of good leakdown test. Certainly isn't the clutch, transmission, or cv joints because it can occur when the car is stopped. Consider leaking cylinder head gasket as this was replaced by the same shop that screwed up my variocam? Timing chain rattle? There's no oil in the coolant or vice versa. Please don't say scored bores!

Here's another great article on engine noise.

 

If you found such a great mechanic, why are you asking us? He would probably be the best equipped to answer your question, especially with all the stuff that has happened so far.

 

It turns out it was a bad cylinder head installation. The shop that screwed up my 1-3 variocam also happened to screw up the bank 4-6 cylinder head; they had re-used the old bolts.

As the problem got worse, my new mechanic found exhaust and coolant sputtering out of the bank 4-6. Time to drop the engine yet again. Argh!

 

I hope you got a great deal on this car.... And that you get your problems fixed. I wouldn't drive it a block with any metal on metal noises.