'97 540 problem

[bmwbill]

First post here--hopefully somebody out there can help as I'm automotively challenged.

I've noticed that occassionally when it rains (heavy), I get a loud knocking sound that seems to come from under the car, roughly under the center console. This happens only at low (under 30 mph) speeds. I can actually feel the knocking vibration on the center console. This sound comes and goes. I've never experienced it when it's dry and only occassionally when its VERY wet.

The car has 82,000 miles, is well maintained, and has hi-test (91 octane) in the tank.

What do you think it is? Is it safe to drive? (My wife just called me, she drove two hours on the highway, and as soon as she got on back roads she called me and said the car is making this noise. I am assuming she's hearing the same thing I've occassionally heard in the past.

All replies appreciated. Thanks in advance.

Bill

[billB]

BMWBILL

can you be more specific on this knocking sounds? sounds like what? metal high pitch loud ticking etc etc let us forget about wet/dry for a second.

let us know we will get back to you ASAP

[bmwbill]

Thanks for the reply--

Sounds like a knuckle rapping on a very solid door, frequency two or three times a second, does not vary with rpm (I think). I would put the sound at about 100hz.

Is it possible that I have a loose clamp on the exhaust system and driving through a deep puddle cools off the pipes enough that they contract, and therefore vibrate?

[billB]

Based on your description I think you are safe...next oil change have your guy check under for loose bolt/shield/clamp/anything loose . If this is not the answer we will try step two....not a biggy.....enjoy your car they are good and fun to drive

Three Notes:

A. There is a big difference between a "knocking" sound (as you describe it) and a "ticking" sound which is used to describe lifter noise. Since you don't feel this noise is coming from the engine and/or lifters I'm assuming it really isn't a "ticking" sound. By the way, a knock sound is usually associated with a part that has some bulky mass or heaviness to it which lifters lack.

B. BMWs are very famous with TICKING NOISE after you turn them off although it is not as loud as "knocking" on a solid door, you still hear ticking. That is your converter cooling down...while at it try to relate this to the wet/dry situation you have.....I tend to doubt that is the case however.

C. If I have to put some money I'll bet on a heat shield. BMWs have a rather extensive thin metal sheet running over the exhaust system and by the drive shaft. These sheets are kept in place by small screws; they tend to rust and hence loose matal knocking under.....make sure someone check it out for you. The car has to be lifted by a mechanic/oil change place etc etc and don't go under it.
let us know and good luck
regards
billb

[bmwbill]

Thanks. Bill

[///Matt]

I have the EXACT same problem on my 97' 540ia...Noticed it right after a carwash. I think it is connected to water actually getting up the exhaust to a point that, when turned to steam, pressure builds up and "bubbles" out what water is left making that low rumbling sound. At first i thought it was my tranny going out...I haven't had a relapse of it lately, but monday the car goes in for Inspection II, so if there's a problem they'll catch it...

-MW