Noise in the cab at one rpm only.

I have a 1988 Toyota, carbureted 2WD pickup. The truck has a very quiet
exhaust system but at a low rpm just above idle it makes an annoying
"vroom" sound in the cab each time it passes through that rpm. If that
rpm is held steady it will continue. There is no such sound at any
other engine speed. The truck need not be moving. It is so annoying
because that rpm occurs so much in city driving that the "vroom" is
coming and going constantly. It's hard to guess, but the rpm seems to
be around 1000. Maybe a little higher.
I've searched the groups for advice and thus have jacked up the engine
while watching all the motor mounts, and all the following. The exhaust
system is in good shape and properly supported. My efforts to locate
the source of this noise have been mostly by doing things that would
stop the noise so I could figure out what to do. I have weighted the
exhaust pipe at different places to stop any resonating(?) I also put
the jack under it for the same test. It had no affect.
I hooked a hoist to the front of the engine and pulled it up hard, but
it didn't help. I weighted the engine down about a hundred pounds to no
avail. I took off the A.C. and smog pump belt. That made no difference.
I held the fan still while running the engine. No help. The 22r just
seems to love to vibrate at that rpm and it sounds like someone beating
a drum in the cab. Something is resonating. Any suggestions? I am so
stumped that the only other thing I could think of was to come in the
house and join this newsgroup and ask you. It's my first post.
Thanks for any advice you have.

Greetings,

How about a loose baffle plate in the muffler or a loose heat shield
between the muffler and cab floor? Or maybe a heat shield in the engine
compartment...

Bill

"zlen357" <h_l_greene@yahoo.com> wrote in message
news:1137022672.337964.84840@o13g2000cwo.googlegroups.com...[color=blue]
>I have a 1988 Toyota, carbureted 2WD pickup. The truck has a very quiet
> exhaust system but at a low rpm just above idle it makes an annoying
> "vroom" sound in the cab each time it passes through that rpm. If that
> rpm is held steady it will continue. There is no such sound at any
> other engine speed. The truck need not be moving. It is so annoying
> because that rpm occurs so much in city driving that the "vroom" is
> coming and going constantly. It's hard to guess, but the rpm seems to
> be around 1000. Maybe a little higher.
> I've searched the groups for advice and thus have jacked up the engine
> while watching all the motor mounts, and all the following. The exhaust
> system is in good shape and properly supported. My efforts to locate
> the source of this noise have been mostly by doing things that would
> stop the noise so I could figure out what to do. I have weighted the
> exhaust pipe at different places to stop any resonating(?) I also put
> the jack under it for the same test. It had no affect.
> I hooked a hoist to the front of the engine and pulled it up hard, but
> it didn't help. I weighted the engine down about a hundred pounds to no
> avail. I took off the A.C. and smog pump belt. That made no difference.
> I held the fan still while running the engine. No help. The 22r just
> seems to love to vibrate at that rpm and it sounds like someone beating
> a drum in the cab. Something is resonating. Any suggestions? I am so
> stumped that the only other thing I could think of was to come in the
> house and join this newsgroup and ask you. It's my first post.
> Thanks for any advice you have.
>[/color]

You might try pulling the vacuum hose off the EGR valve and plugging it
temporarily as a test. Sometimes they can make noise.

Q

zlen357 wrote:[color=blue]
> I have a 1988 Toyota, carbureted 2WD pickup. The truck has a very quiet
> exhaust system but at a low rpm just above idle it makes an annoying
> "vroom" sound in the cab each time it passes through that rpm. If that
> rpm is held steady it will continue. There is no such sound at any
> other engine speed. The truck need not be moving. It is so annoying
> because that rpm occurs so much in city driving that the "vroom" is
> coming and going constantly. It's hard to guess, but the rpm seems to
> be around 1000. Maybe a little higher.
> I've searched the groups for advice and thus have jacked up the engine
> while watching all the motor mounts, and all the following. The exhaust
> system is in good shape and properly supported. My efforts to locate
> the source of this noise have been mostly by doing things that would
> stop the noise so I could figure out what to do. I have weighted the
> exhaust pipe at different places to stop any resonating(?) I also put
> the jack under it for the same test. It had no affect.
> I hooked a hoist to the front of the engine and pulled it up hard, but
> it didn't help. I weighted the engine down about a hundred pounds to no
> avail. I took off the A.C. and smog pump belt. That made no difference.
> I held the fan still while running the engine. No help. The 22r just
> seems to love to vibrate at that rpm and it sounds like someone beating
> a drum in the cab. Something is resonating. Any suggestions? I am so
> stumped that the only other thing I could think of was to come in the
> house and join this newsgroup and ask you. It's my first post.
> Thanks for any advice you have.
>[/color]

I had a similar problem on my 85 toy- turned out to be a cannister in the
engine compartment( on the intake side of the motor) . The evaporative
cannister base is close to a notch of the inner fender. I was working on the
engine and might have put enough pressure on the can to bend it slightly,
pushing it close to the notch which it hit when the rpm was sympathetic. Pat

Bill Pelka wrote:[color=blue]
> Greetings,
>
> How about a loose baffle plate in the muffler or a loose heat shield
> between the muffler and cab floor? Or maybe a heat shield in the
> engine compartment...
>
> Bill[/color]

Some catalytic converters have a heat shield too and can get loose enough to
vibrate.
davidj92