Someone came to look at my 100% stock 1995 VX (127K miles) who was interested in possibly buying it. I know, I know, why would anyone sell their VX? The plan was to use it as seed money for a Honda Insight.
That said, we took it to the buyer's mechanic for a quick eval before he made the purchase. I wasn't worried. The car runs perfect and has been perfectly maintained. I can get 50 MPG with a CA car without too much problem. The mechanic thought he heard one of the engine main bearing making a noise.
At a specific RPM the engine makes a little bit of a deeper noise. Something like a light thud, thud, thud in the background with the normal valve noise in the foreground. It is at about 1,400 rpm and goes away if the RPM is changed 200 RPM either way.
Is there anyway this is really a bearing? How good are the VX bottom ends? 127K miles seems pretty young to be having such a problem. The mechanic gave it 10K miles to failure. I was surprised to say the least and the buyer and I are going for a second opinion. Anyone have a thought?
Usually, bearing noise will be worse during the first 2 seconds of a cold start or for the 3-4 seconds immediatley after changing the oil and filter (while the oil filter is filling). You will hear it knocking more loudly until the oil pressure light goes off.
Another mechanics trick is to take a wooden broom handle (no broom on the end) and place one end aganist your ear and the other end against different places on the engine block. You should be able to isolate the location of the noise this way.
Check the oil and make sure the owner doesn't have extra thick oil in it to help cover up the loose bearing noise. (whoops- you are the owner so you would know what weight of oil you were using)
Lug the engine down to 200-300 rpm or lower when hot by putting the engine in 5th gear and riding the clutch briefly. If the oil light comes on while the engin eis still turning, this means that the oil pressure is only marginal and bearings are likely worn. Note: the engine should be hot for this test (the oil will be nice and thin)
Another possibility is piston slap (excessive piston/cylinder wall clearance). This is worse when accelerateing when cold and gets more faint as the engine warms. Some piston slap is pretty common and isn't much to worry about unless its severe.
I took the car to the dealer and one of the long term Honda techs said there was nothing wrong with the engine. After all the effort he didn't purchase the car anyway. Looks like the Insight will have to wait for now.
I'm going to assume that you don't hear it louder when it's cold because you said it was the first time you'd heard it.
Since it happens at one rpm and not all, the likelihood of it being a bearing is VERY slim. Bearings are usually never picky as to what rpm they sound like crap at. It's probably more like a heat shield or something else along those lines. How fast is the sound?
My Suzuki J20 has harmonic issues that makes the engine vibrate harshly if loaded at 1700-1800rpm but that causes something you can feel and it still pulls and sounds great past that all the way to the 6600 fuel cut.
Hearing a main bearing making noise is a pretty neat trick considering that the crank would have to flex enough for the bearing surface to hit the bearing sleave with enough force to get through the oil film and make some noise. More likely a little piston slap or possibly a little rod knock since you are running a light load and probably get the timing max advanced under those conditions. Checking the running oil pressure from cold to hot and at idle would tell you how badly the bearings are worn by how low the oil pressure gets. More likely you are getting some resonance in the valve train between the cam chain and cam shaft making the belt/chain slap around a little and it is nothing to worry about.