Strange whooping sound

[Azur]

Dear,
Last week and yesterday for the first time a strange sound heard from the engine room on the driver's side. Last week for the first time a caravan pulled with the mk1 ppd 2.5V6 R 75, driven only a few kilometers. After I disconnected the caravan and restarted the engine, I heard a pretty loud sound like a rising siren. It started with a low loud tone which increased to a high tone and then stopped. Surrounding sound driver's side RHD from engine compartment. Sound lasted about 2 to 3 seconds and then stopped. No further problems with driving. Yesterday I drove again with a high outside temperature, and after restarting again this loud rising sound, same principle. No problems with driving, brakes or air conditioning. What can this be? Some kind of venting of something? What should I look at?

Thanks in advance for the reactions.

Kind regards Emiel

[roverbarmy]

Sound like the hot weather has activated the fan onto higher speed/s. Once cooled it will switch off or slow down. Often happens in standing traffic. Hopefully, you have no other cooling problems or coolant loss.

[Azur]

Hi, thanks, no, I recognise that sound, that is completely different and a normal sound. No cooling problems at all luckely. This is really a weird "slow whoop" sound and should not be there. Anybody?

[Teflon]

Did it sound a bit like the noise a whale makes? If so, it's the air con, and is a fairly well known phenomenom. The cause is much debated, and it may indiacte that a re-gais is needed, but unless it does it too frequently, I personally wouldn't wory too much.

When I bought my '75 some 14 yrs ago, it made the noise every time I switched the aircon on for the first few weeks, then it disappeared completely. Over the intervening years, my aircon has been serviced numerous times, including fitting a brand new fan, but still very occassionally makes the noise on a very hot day, perhaps maybe once or twice a year if that

Cliff

[roverbarmy]

Originally Posted by Azur

Hi, thanks, no, I recognise that sound, that is completely different and a normal sound. No cooling problems at all luckely. This is really a weird "slow whoop" sound and should not be there. Anybody?

Ah! The whale whoop. As Teflon says, you may need an aircon check.

[Azur]

Thanks both for your reactions. I will have a check on that!

Regards Emiel

[T-Cut]

To quote member Ambleson - the whooping sound is from thermal transfer valve (TRV) reacting to the temperature drop in the evaporator. The evaporator is supplied with liquid refrigerant via the TRV and if the temperature in the evaporator is higher than 4°C the TRV opens to allow more gas to evaporate and so cool the unit. The evaporator starts cooling and the TRV detects this and slows down the amount of liquid released. It is this shut down that produces the whooping noise until the equilibrium is reached. The slight hiss you may hear is the liquid refrigerant boiling off in the evaporator. This produces the cooling. Press the 'Econ' button and the hiss will stop. The evaporated gas is then compressed and cooled in the condenser at the front of the car. This stage requires the slow running radiator fan to produce the liquid refrigerant again.

To add to this, the efficiency of the condenser is also important and any insufficiency there will also contribute to the whooping noise. If a simple recharge doesn't resolve the issue, check the condenser. The fins on the matrix are known to disintegrate after several years.

TC

[trikey]

Do not run the aircon until the issue is repaired Emiel, the compressor is over pressurising that is why it’s making the whale sounds!

[Azur]

Thanks guys, your reactions are prooving what a great forum this is! I will go to an airco specialist for a refill and check. @Andy: I will not use it untill checked!
Have a great day you all.

Emiel

[Parker]

My 75 had the Whale whoop all the time I owned it despite having the aircon serviced and gassed.

It is a feature