Inconsistent knock/rattle from engine compartment 2002 Camry 4-cylinder

[cn_sanantonio]

I own a 2002 Camry XLE 4-cylinder with about 95,000 miles which is in excellent condition and well-maintained. Recently I began to hear a rattle, tapping or knocking sound apparently from the passenger side of the engine compartment (passengers agree with apparent location). It almost sounds like engine knocking, but the times it happens seem to rule that out. It could be some metal vibrating against something, but I can't hear it when I'm under the hood, and nothing appears loose. I've taken it to my trusted independent mechanic, but he couldn't duplicate the sound driving for 15-20 minutes on the freeway or see anything wrong when put on a lift. The sound doesn't occur all the time, of course; that'd be too easy.

The noise begins:

• after I've been driving 15 minutes or more
• after the car gets above 45-50 MPH, but when the noise finally begins, it will then occur down to as low as 30 MPH. Last weekend I drove 1000 miles roundtrip on the freeway at 65-80 MPH; it didn't begin for 30 minutes, but continued most of the way to my destination (5.5 more hours), though it did seem to stop a few times. This week, my 20 minute drive to work, which has about 10 minutes on the freeway at 65, has not caused the noise to happen once.

Characteristics of the problem:

• the noise is not related to RPM; once it begins, it is about the same frequency and volume at 30 MPH as at 80 MPH
• it doesn't occur when in park or neutral and idling, even if the engine is revved, and even if the noise was just occurring on the freeway moments before
• it doesn't change when running over rough pavement or in turns
• it doesn't change when regular gas is replaced with premium (tried this last week to rule out engine knock, though the lack of RPM affecting it probably ruled that out already).

So it seems like it may be related to the speed of the car and perhaps the temperature of the engine or compartment. The hood latch appears to be holding tight with no play, the strut mounts were replaced 10K ago, and the wheel bearings check out fine.

I've read on the forum about manifold noise in this model, but almost everyone wrote about it occurring at low speeds or even idle. Has anyone had a similar issue they figured out? Or does anyone have other suggestions for what I can check? I appreciate any advice!

Thanks,

cn

[BlackCamSe]

Look under the front of the car and check the cv boots. Does it do it worse while making turns?The only other thing I can think of is worn swaybar bushings or bad endlinks.

[cn_sanantonio]

Originally Posted by BlackCamSe

Look under the front of the car and check the cv boots. Does it do it worse while making turns?The only other thing I can think of is worn swaybar bushings or bad endlinks.

CV boots look okay. I don't notice the noise in turns, but then I'm rarely doing 30+ mph when turning. (I drive fairly conservatively and basic 2002 suspension is mushy. ) Do you think the sway bar would make more noise only at higher speeds in a warmed-up car?

Thanks!

[Tech1984]

I'm betting it's the serpentine belt tensioner. The damper gets weak and a "flutter" sound develops on initial take off.
With the e-brake applied, in drive, foot on brake, try tapping the gas pedal quickly and it's easy to duplicate.

[cn_sanantonio]

Thanks for the suggestion, but I don't hear the noise at all when revving the engine quickly with the transmission in Drive and the brake applied.