Sounds like a box of rocks

Between 2k and 2500 rpm, it sounds like a box of rocks. I don't know how else to describe it. Only when there's a load on the engine will it make this noise. If I'm maintaining speed in this rpm range, it's fine. No noise at idle or any other rpm. It's not quite like spark knock. I can't find anything loose in the exhaust, no CEL, I tried premium gas in case it was knock, no change. With these conditions, I don't know what else it could be. New plugs, wires, fuel filter, ecm recal for summer fuel in cold temps, oil changed regularly, 105k mi, OE timing belt.

Between 2k and 2500 rpm, it sounds like a box of rocks. I don't know how else to describe it. Only when there's a load on the engine will it make this noise. If I'm maintaining speed in this rpm range, it's fine. No noise at idle or any other rpm. It's not quite like spark knock. I can't find anything loose in the exhaust, no CEL, I tried premium gas in case it was knock, no change. With these conditions, I don't know what else it could be. New plugs, wires, fuel filter, ecm recal for summer fuel in cold temps, oil changed regularly, 105k mi, OE timing belt.

What year ZX2???

I'll assume you are sure that it is an ENGINE SPEED (RPM) issue and not a VEHICLE SPEED (MPH) problem. That said I would suspect a worn bearing in one of the accessories that rotates at engine speed: alternator, AC compressor, power steering, water pump, or one of the idler/tensioner pulleys.

They are located as shown in this link (scroll down to bottom):
http://www.rldassociates.com/zx2/howto/belt.htm

I would remove the accesory drive belt and turn each accessory/pulley by hand (some may be tough to reach). Ill bet you will find one that feels 'rough' - that will most likely be the one with the bad bearing. In fact another good test would be to remove the accesory drive belt and start the engine, bring it to the RPM you normally hear it at and listen for the noise. If its gone then you have verified that its in one of the accessories/pulleys. You can run the engine for short periods of time with the belt removed without causing any problems.

Good luck,
Paul

It's definitely not a speed issue. The vehicle is a 98. The Idler/tensioner, whatever it was got replaced about two years ago. Thanks for the belt removal idea, I'll try that. Do you think an accessory could cause this? It really only happens under acceleration.

What year ZX2???

I'll assume you are sure that it is an ENGINE SPEED (RPM) issue and not a VEHICLE SPEED (MPH) problem. That said I would suspect a worn bearing in one of the accessories that rotates at engine speed: alternator, AC compressor, power steering, water pump, or one of the idler/tensioner pulleys.

They are located as shown in this link (scroll down to bottom):
http://www.rldassociates.com/zx2/howto/belt.htm

I would remove the accesory drive belt and turn each accessory/pulley by hand (some may be tough to reach). Ill bet you will find one that feels 'rough' - that will most likely be the one with the bad bearing. In fact another good test would be to remove the accesory drive belt and start the engine, bring it to the RPM you normally hear it at and listen for the noise. If its gone then you have verified that its in one of the accessories/pulleys. You can run the engine for short periods of time with the belt removed without causing any problems.

Good luck,
Paul

It's definitely not a speed issue. The vehicle is a 98. The Idler/tensioner, whatever it was got replaced about two years ago. Thanks for the belt removal idea, I'll try that. Do you think an accessory could cause this? It really only happens under acceleration.

Yes definately, either an accessory or one of the idler pulleys. FYI Mustang 5.0L engines are notorius for getting exactly this type of problem from their belt tensioners. A few years ago I had an alternator go bad and it hada bad bearing that was noisy at certain RPMs.

Paul

ps
Please click here to check out my 78 Zephyr at my crappy Cardomain webpage (http://www.cardomain.com/ride/773108/1)

I have the same problem, but im almost certain that is is a damaged flexplate (auto flywheel). If its the same for you than you'll notice that it only makes that god awful sound is when it is in drive, but when it's in park or neutral you can redline the engine and it wont make the noise. Hope this helps.