The motor on my 89' Accord all of a sudden seems much louder to me under load almost as if there was a hole in the muffler. Idleing and cruising it is as quiet as it always was, but when in gear and giving it gas I can hear that chug, chug, chug sound of the engine working much louder then I used to. I replaced the entire exhaust (other then the manifold) about 6 months back and I can not feel any leak at any of the joints. I am guessing it may be a valve tap but I am not really sure. I changed the oil about a month ago and the checked the level yesterday (doesn't need a drop).
I know this is a kind of vague question, but it there anything I can do as a preventative measure? Things to check? Maybe it is just all in my head...

 

Exhaust manifold gasket (against the head) leaking??
(maybe it IS in your head...)

Other joints you may not have checked?
Cat flanges?
O2 sensor leaking?

 

I checked all around the exhaust manifold and could not feel any air movement. The cat is new and I could not feel or hear any leak at the flanges. One other observation to note; I can rev the engine when it is not under load (in neutral in my driveway) and it is does not sound nearly as loud as when I am driving. Going up a hill it is even more noticable.

 

After replacing exhaust parts, maybe it's the alignment of all the exhaust hangers.

When climbing a hill the engine rocks backwards in it's mounts as it's delivering more power. That can move the whole exhaust system, & maybe something hits against the floor-pan.

Speaking of the exhaust system moving around, maybe that movement causes a leak to open up?? You may be able to duplicate it by quickly goosing the gas pedal or even shaking the engine?

Or maybe you have a bad engine mount? Rubber part is deteriorated, and the metal parts of the mount come together & transmit the engine vibration to the car's structure.