Hey all. Recently, my old 1993 Accord LX has developed a few small nagging issues that are bugging me, and I was hoping you guys could help me sort her out.

About one month ago, I noticed that when I accelerated hard from a stop, when my engine reached 2100 RPM under load, a quiet chattering noise could be heard from the driver's side of the vehicle. The noise is hard to describe, but the closest I could come to would be someone tipping a plastic container of marbles. The weird thing about the issue is that is simple to replicate, but very particular. It only happens when the engine is still not fully warm (as in, driven less than ten miles). It also ONLY happens at 2100 RPM. Not 2000, not 2200, but 2100. It will chatter as long as I hold that RPM, but go beyond or under and it stops immediately. Finally, it usually only occurs if I am accelerating relatively hard. A gentle takeoff will result in a quiet run.

Yesterday, a new and more pressing issue arose. I just took it on a long, hilly freeway drive for the first time in several months. Along the trip, I noticed that the vehicle has started shuddering when I accelerate hard from a dead stop. The shudder is not massive, but definitely pronounced over the normal action of the engine. It only occurs for a moment and only just after starting to accelerate-I originally thought that the crosswalks in the area I was visiting were oddly bumpy, as the shuddering is more like one or two hard but very short spasms, as if my front wheels were going over a bump in the road. It also ONLY occurs when I accelerate hard, but ALWAYS occurs when I do so. There are no noticeable RPM fluctuations or loss of power when this happens-I can still drive up a hill from a dead stop.

The engine performs flawlessly under all other circumstances-no misfiring, no transmission slippage, smooth shifting, smooth idle, proper mileage, etc. Transmission has 210,000 miles and had a fluid change with genuine Honda ATF-Z1 about 5000 miles ago. No CEL codes, and engine timing appears to be within specs (consistent red line at marker). Hopefully you guys will point out something I am missing, because I'm completely lost as of now...

 

My first guess is the cat is starting to go bad. That would explain the marble sound(cat has disentegrated) and your hesitation(blockage).

 

Possibly EGR? Detonation and pinging can sound like valvetrain chatter. The heavier the load the louder the pinging will be, gently drive away not much noise, heavy acceleration(not WOT) more pinging.

If the car has over 200K miles and the EGR has never been cleaned it could be partially clogged and is starting to affect engine performance.
May try by removing the vacuum hose to the EGR valve, plugging it, and then going for a drive. It should set a code for insufficient EGR lift/EGR solenoid, but before it does that you may be able to increase the noise before the ECU corrects or goes into limp mode. If it is that same noise, then it may be time to clean out the EGR ports in the intake.